Trimmed belly: about 45 inches! (3)
I believe the answer is:
ell
'45 inches' is the definition.
(I've seen this before)
'trimmed belly about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'ELL' can be found hidden inside 'trimmed belly'.
(Other definitions for ell that I've seen before include "Tailor's measure" , "45 inches (cloth)" , "Old 45-inch measure" , "Extension" , "Former measure of length" .)