Wellington is smooth, fitting round front of knee (4,4)
I believe the answer is:
iron duke
'wellington' is the definition.
(I've seen this before)
'smooth fitting round front of knee' is the wordplay.
'smooth' becomes 'iron' (I've seen this before).
'fitting' becomes 'due'.
'round' is an insertion indicator.
'front of' suggests taking the first letters.
The initial letter of 'knee' is 'k'.
'due' placed around 'k' is 'duke'.
'iron'+'duke'='IRON DUKE'
'is' is the link.
(Other definitions for iron duke that I've seen before include "Wellington's nickname" , "leader of old" , "Former PM" , "Wellington's byname" , "Waterloo's Wellington was known as the . . . . . . . ." .)