Repair a barrel when in temper (4,4)
I believe the answer is:
make good
'repair' is the definition.
(thesaurus)
'a barrel when in temper' is the wordplay.
'barrel' becomes 'keg' (keg is a kind of barrel).
'when in' is an insertion indicator.
'temper' becomes 'mood' ('mood' can be a synonym of 'temper').
'a'+'keg'='akeg'
'akeg' inserted inside 'mood' is 'MAKE GOOD'.
(Other definitions for make good that I've seen before include "Succeed, become prosperous" , "Repair; do well" , "Repair; be successful" , "Replace (loss); fulfil (promise)" , "Succeed in life, do well" .)