A request certain to gain preference (8)
I believe the answer is:
pleasure
'preference' is the definition.
(thesaurus)
'a request certain' is the wordplay.
'a request' becomes 'plea' (plea is a kind of request).
'certain' becomes 'sure' (synonyms).
'plea'+'sure'='PLEASURE'
'to gain' acts as a link.
(Other definitions for pleasure that I've seen before include "Happy satisfaction" , "Urgent entreaty begins gratification" , "entertainment" , "diversion" , "A very desirable feeling" .)