Half-hearted fast about to be broken by papa (5)
I believe the answer is:
tepid
'half-hearted' is the definition.
(I've seen this before)
'fast about to be broken by papa' is the wordplay.
'fast' becomes 'diet' (diet is a kind of fast).
'about' is a reversal indicator.
'to be broken by' means one lot of letters goes inside another (inserted letters break into the word).
'papa' becomes 'P' (NATO / phonetic alphabet).
'diet' backwards is 'teid'.
'teid' enclosing 'p' is 'TEPID'.
(Other definitions for tepid that I've seen before include "without passion" , "Luke warm" , "Half-hearted" , "Not very warm (water)" , "Of liquid, luke-warm" .)