Backside of prominent Liberal observed in mature threesome (6)
I believe the answer is:
triple
'threesome' is the definition.
(thesaurus)
'backside of prominent liberal observed in mature' is the wordplay.
'backside of' says to take the final letters.
'liberal' becomes 'l' (abbreviation).
'observed in' indicates putting letters inside.
'mature' becomes 'ripe' (similar in meaning).
The last letter of 'prominent' is 't'.
'l' placed within 'ripe' is 'riple'.
't'+'riple'='TRIPLE'
(Other definitions for triple that I've seen before include "Let rip with a threesome" , "Three times" , "In three parts" , "Multiply by three" , "make score sixty?" .)