'Wannabe' when playing in part? (8)
I believe the answer is:
aspirant
'wannabe' is the definition.
('aspirant' can be a synonym of 'wannabe')
'when playing in part?' is the wordplay.
'when' becomes 'as'.
'playing' is an anagram indicator.
'in'+'part'='inpart'
'inpart' with letters rearranged gives 'pirant'.
'as'+'pirant'='ASPIRANT'
(Other definitions for aspirant that I've seen before include "One is working hard to achieve something" , "One who wants to succeed" , "Awannabee found in past rain" , "Partisan may be person with high ambition" , "One seeking eg promotion" .)