Bet success will be welcomed by a theatre fan (8)
I believe the answer is:
playgoer
'a theatre fan' is the definition.
(I've seen this before)
'bet success will be welcomed' is the wordplay.
'bet' becomes 'lay' (lay bet is a type of bet in gambling).
'success' becomes 'go' (I've seen this in other clues).
'will be' becomes 'per' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'welcomed' is an insertion indicator.
'lay'+'go'='laygo'
'laygo' put into 'per' is 'PLAYGOER'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for playgoer that I've seen before include "Theatre fan" , "One who's regularly seen in theatre" , "Theatre patron" , "Person in theatre" , "He likes the Theatre" .)