Gannet gets climbing part as a practical joke (5,4)
I believe the answer is:
booby trap
'a practical joke' is the definition.
(I've seen this before)
'gannet gets climbing part' is the wordplay.
'gannet' becomes 'booby' (booby is a kind of gannet).
'gets' means one lot of letters go next to another.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'part' in reverse letter order is 'trap'.
'booby'+'trap'='BOOBY-TRAP'
'as' is the link.
(Other definitions for booby trap that I've seen before include "hidden hazard" , "Unexpected explosive device" , "Lethal or playful device set off by victim" , "Bra, maybe" , "Hidden weapon" .)