Mature priest sitting on monstrous female (8)
I believe the answer is:
progress
'mature' is the definition.
(thesaurus)
'priest sitting on monstrous female' is the wordplay.
'priest' becomes 'pr'.
'sitting on' means one lot of letters go next to another.
'monstrous female' becomes 'ogress' (I've seen this before).
'pr'+'ogress'='PROGRESS'
(Other definitions for progress that I've seen before include "Move forward" , "Forward movement towards a destination or goal" , "Go on - journey" , "Advancement" , "Headway" .)