In retreat, keep following banned subject for trade (5,5)
I believe the answer is:
ivory tower
'in retreat' is the definition.
'ivory tower' can be an answer for 'retreat' (I've seen this in another clue). I'm not certain of the 'in' bit.
'keep following banned subject for trade' is the wordplay.
'keep' becomes 'tower' (keep can mean a fortified structure).
'following' says to put letters next to each other.
'banned subject for trade' becomes 'ivory' (as in the ivory trade).
'tower' put after 'ivory' is 'IVORY TOWER'.
(Other definitions for ivory tower that I've seen before include "State of seclusion from worldly realities" , "Seclusion from ordinary life" , "Academia (seen as secluded)" , "academic position" .)