Instantly memorable (6)
I believe the answer is:
catchy
'instantly memorable' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for catchy that I've seen before include "(Of a tune) instantly appealing and memorable" , "Type of tune that stays in the memory" , "Notedly attractive" , "Of tune, lilting and easy to remember" , "Easy to remember and sing" .)