No. A preposition never takes past tense, so looked cannot be one. In She looked at the clock, looked is the past tense of the verb look, and at is a separate preposition sitting after it. The same pattern holds in phrasal verb phrases: words like at, for and after follow looked, while looked itself stays the verb.
No. Looked is a verb form, and it stays one in every sentence it appears in. The reason is simple: prepositions never take past tense. A word like at or for has no past form at all, so any word that shows past tense, such as looked, must be a verb. Take She looked at the clock. Here looked is the verb, the past tense of look, and at is a separate preposition, the second word. Sentences like this can be confusing because a preposition sits right next to the verb, but the position does not change the word classes: the first word carries the tense, and the second word does not.
No. Looked is always a verb form of look, never a preposition. The reason is simple: prepositions never take past tense, so a word carrying past tense like looked cannot be one. In She looked at the clock, looked is the verb and at is a separate preposition, the second word. In phrasal verb patterns, words like at, for and after follow looked, while looked itself stays the verb.