Like an earworm (6)
I believe the answer is:
catchy
'like an earworm' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for catchy that I've seen before include "Attention-grabbing" , "Easy to remember and sing" , "(Of a tune) instantly appealing and memorable" , "Staying in the mind of a tune" , "Tuneful and easily remembered" .)