Change commercial about knights perhaps (5)
I believe the answer is:
amend
'change' is the definition.
(amending is a kind of changing)
'commercial about knights perhaps' is the wordplay.
'commercial' becomes 'ad'.
'about' indicates putting letters inside.
'knights perhaps' becomes 'men' (knight is a kind of man).
'ad' going around 'men' is 'AMEND'.
(Other definitions for amend that I've seen before include "Make small changes to" , "Make minor changes to" , "Make corrections to (eg, document)" , "Make minor improvements to (a document)" , "Change, alter" .)