Part of an amusing turn, that’s all (3)
I believe the answer is:
sum
'all' is the definition.
(thesaurus)
'part of an amusing turn' is the wordplay.
'part of' indicates a hidden word.
'turn' is a reversal indicator.
'mus' can be found hidden inside 'amusing'.
'mus' back-to-front is 'SUM'.
'that's' acts as a link.
(Other definitions for sum that I've seen before include "Whole" , "Arithmetic calculation" , "Total, add" , "All one has" , "pupils find it a problem" .)