Boxer wet leg and I threw wobbly! (12)
I believe the answer is:
welterweight
'boxer' is the definition.
(I've seen this before)
'wet leg and i threw wobbly' is the wordplay.
'and' says to put letters next to each other.
'wobbly' indicates an anagram.
'wet'+'leg'+'i'+'threw'='wetlegithrew'
'wetlegithrew' with letters rearranged gives 'WELTERWEIGHT'.
(Other definitions for welterweight that I've seen before include "Boxing category" , "Boxer (147 lbs max)" .)