The French forward is wearing old lady's slip (11)
I believe the answer is:
malapropism
'slip' is the definition.
(type of linguistic error)
'the french forward is wearing old lady's' is the wordplay.
'the french' becomes 'la'.
'forward' becomes 'prop' (I've seen this before).
'wearing' means one lot of letters goes inside another.
'old lady' becomes 'mam'.
'la'+'prop'+'is'='lapropis'
'lapropis' going inside 'mam' is 'MALAPROPISM'.
(Other definitions for malapropism that I've seen before include "Unintentional confusion of one word with another (by someone in a Sheridan play?)" , "Verbal misapplication" , "Blunder in theatre" .)