Do gymnastics, taken in by pathetic spoilsport (5,6)
I believe the answer is:
party pooper
'spoilsport' is the definition.
('party pooper' can be a synonym of 'spoilsport')
'do gymnastics taken in by pathetic' is the wordplay.
'do' becomes 'party' (I've seen this before).
'gymnastics' becomes 'pe' (Physical Education).
'taken in by' is an insertion indicator.
'pathetic' becomes 'poor' ('poor' can be a synonym of 'pathetic').
'pe' inserted inside 'poor' is 'pooper'.
'party'+'pooper'='PARTY-POOPER'
(Other definitions for party pooper that I've seen before include "Social 1 across" , "Spoilsport" , "One spoiling others' fun" , "One who spoils a social occasion" .)