Half-hearted gospel minister’s back on with a mass (8)
I believe the answer is:
lukewarm
'half-hearted' is the definition.
(synonyms)
'gospel minister's back on with a mass' is the wordplay.
'gospel' becomes 'Luke' (book of the New Testament).
'minister's back' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'with' becomes 'w' (abbreviation).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'w'+'a'='wa'
'r' after 'wa' is 'war'.
'luke'+'war'+'m'='LUKEWARM'
Can you help me to learn more?
(Other definitions for lukewarm that I've seen before include "Unenthusiastic" , "Somewhat cooler!" , "Not enthusiastic, neither hot nor cold" , "Moist" , "Not enthusiastic like tepid water" .)