First one off to get a tan? Better take top off (5)
I believe the answer is:
uncap
'take top off' is the definition.
(I've seen this before)
'first one off to get a tan? better' is the wordplay.
'first one off' means to remove the first letter.
'to get a tan?' becomes 'sun'.
'better' becomes 'cap' (I've seen this before**).
'sun' with its first letter removed is 'un'.
'un'+'cap'='UNCAP'
(Other definitions for uncap that I've seen before include "Remove a top" , "Take the lid off" , "Remove a cover from" , "Remove the lid" , "Take the top of the bottle" .)