Good leg exercise? Not beginner's messy stuff (5)
I believe the answer is:
gunge
'messy stuff' is the definition.
'good leg exercise? not beginner's' is the wordplay.
'good' becomes 'g' (abbreviation).
'leg exercise?' becomes 'lunge'.
'not' is a deletion indicator.
'beginner' becomes 'l'.
'lunge' with 'l' taken out is 'unge'.
'g'+'unge'='GUNGE'
(Other definitions for gunge that I've seen before include "Unpleasant thick substance" , "Unpleasant sticky material" , "Dirty substance" , "Congealed matter" , "Thick sticky muck" .)