Will appeal, certainly (8)
I believe the answer is:
pleasure
'will' is the definition.
(I've seen this before)
'appeal certainly' is the wordplay.
'appeal' becomes 'plea' (plea is a kind of appeal).
'certainly' becomes 'sure' (synonyms).
'plea'+'sure'='PLEASURE'
(Other definitions for pleasure that I've seen before include "A desirable emotion" , "A very desirable feeling" , "Urgent entreaty begins gratification" , "amusement" , "'Enjoyment, delight (8)'" .)