Decline of French commander English think highly of? (10)
I believe the answer is:
degenerate
'decline' is the definition.
(degenerating is a kind of declining)
'of french commander english think highly of?' is the wordplay.
'of french' becomes 'de'.
'commander' becomes 'gen' (abbreviation for general).
'english' becomes 'e' (abbreviation).
'think highly of?' becomes 'rate' (I've seen this before).
'de'+'gen'+'e'+'rate'='DEGENERATE'
(Other definitions for degenerate that I've seen before include "Rogue" , "Unprincipled" , "base" , "Decline physically or morally" , "Decline, deteriorate" .)