Apathetic leader of party in regime overthrown (5)
I believe the answer is:
tepid
'apathetic' is the definition.
(thesaurus)
'leader of party in regime overthrown' is the wordplay.
'leader of' indicates taking the first letters.
'in' is an insertion indicator.
'regime' becomes 'diet' (eating regime).
'overthrown' shows that the letters should be reversed in order.
The first letter of 'party' is 'p'.
'p' going inside 'diet' is 'dipet'.
'dipet' back-to-front is 'TEPID'.
(Other definitions for tepid that I've seen before include "What's a bit warm" , "Not very warm (water)" , "without enthusiasm" , "without passion" , "Showing little interest or enthusiasm" .)