Most enormous rat's leg, broken (7)
I believe the answer is:
largest
'most enormous' is the definition.
('large' can be similar in meaning to 'enormous')
'rat's leg broken' is the wordplay.
'broken' indicates anagramming the letters.
'rats'+'leg'='ratsleg'
'ratsleg' anagrammed gives 'LARGEST'.
(Other definitions for largest that I've seen before include "Chief" , "Maximum" , "Of greatest magnitude" , "Biggest" , "Most ample" .)