Happen to gamble on Windies every now and again (6)
I believe the answer is:
betide
'happen to' is the definition.
(synonyms)
'gamble on windies every now and again' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling).
'on' says to put letters next to each other.
'every now and again' indicates alternate letters.
The alternating letters of 'windies' are 'ide'.
'bet'+'ide'='BETIDE'
(Other definitions for betide that I've seen before include "Woe - -" , "Befall" , "Happen to (arch)" .)