Help gambler to choose the odds (4)
I believe the answer is:
abet
'help' is the definition.
(I've seen this before)
'gambler to choose the odds' is the wordplay.
'choose the odds' indicates alternate letters (letters in odd-numbered positions are used).
'gambler'+'to'='gamblerto'
The alternate letters of 'gamblerto' are 'ABET'.
(Other definitions for abet that I've seen before include "Support, aid" , "Encourage or assist (someone) to commit crime" , "Help to do something wrong" , "Help in wrongdoing" , "Aid and assist" .)