Clue: When startled, pets rejected unpleasant modified lettuce, perhaps (6)
Answer: ANNUAL
Explanation: Definition: lettuce, perhaps
An anagram ('modified') of 'un[p]l[e]a[s]an[t]' minus PEST, an anagram ('when startled') of 'pets'. ANNUAL as a plant with a single year life cycle, with 'lettuce' as one example among many.