Happiness certain to follow appeal (8)
I believe the answer is:
pleasure
'happiness' is the definition.
(thesaurus)
'certain to follow appeal' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to follow' means one lot of letters go next to another.
'appeal' becomes 'plea' (plea is a kind of appeal).
'sure' put after 'plea' is 'PLEASURE'.
(Other definitions for pleasure that I've seen before include "A repulse (anag.)" , "Delectation, delight" , "diversion" , "entertainment" , "'Enjoyment, delight (8)'" .)