Excess that is, in retrospect, producing muscles (6)
I believe the answer is:
glutei
'muscles' is the definition.
(gluteus is a kind of muscle)
'excess that is in retrospect' is the wordplay.
'excess' becomes 'glut' (I've seen this before).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'in retrospect' says the letters should be written backwards.
'ie' back-to-front is 'ei'.
'glut'+'ei'='GLUTEI'
'producing' acts as a link.
(Other definitions for glutei that I've seen before include "behind?" , "muscles" .)