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' says the answer is hidden in the clue (the clue goes around or about the answer).
'ELL' is hidden in the letters of '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" .)