Decline betting and run off (5)
I believe the answer is:
spurn
'decline' is the definition.
(I know this)
'betting and run off' is the wordplay.
'betting' becomes 'sp'.
'and' means one lot of letters go next to another.
'off' indicates an anagram.
'run' anagrammed gives 'urn'.
'sp'+'urn'='SPURN'
(Other definitions for spurn that I've seen before include "Slight" , "Reject with contempt" , "Reject scornfully, rebuff" , "Humber lighthouse" , "discard" .)