Spy one-time GP playing on piano (7,3)
I believe the answer is:
peeping tom
'spy' is the definition.
(I've seen this before)
'one-time gp playing on piano' is the wordplay.
'playing' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'piano' becomes 'p' (musical abbreviation).
'one'+'time'+'gp'='onetimegp'
'onetimegp' with letters rearranged gives 'peeingtom'.
'peeingtom' placed around 'p' is 'PEEPING TOM'.
(Other definitions for peeping tom that I've seen before include "Voyeur" , "Spy" , "One who spies pruriently on others" , "who likes looking at females" .)