Wager about a small animal (5)
I believe the answer is:
beast
'animal' is the definition.
(I know that animal can be written as beast)
'wager about a small' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'bet' going around 'as' is 'BEAST'.
(Other definitions for beast that I've seen before include "Large animal -- objectionable person" , "Animal; cruel person" , "Traditional poet" , "Inhumanly cruel person" , "Nasty type" .)