Prayer certain to produce happiness (8)
I believe the answer is:
pleasure
'happiness' is the definition.
(thesaurus)
'prayer certain' is the wordplay.
'prayer' becomes 'plea' (plea is a kind of prayer).
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'plea'+'sure'='PLEASURE'
'to produce' is the link.
(Other definitions for pleasure that I've seen before include "Delight, happiness" , "Urgent entreaty begins gratification" , "Delight, joy" , "A very desirable feeling" , "A repulse (anag.)" .)