Maybe slip beneath English river (9)
I believe the answer is:
underwear
'maybe slip' is the definition.
(slip is a type of underwear)
'beneath english river' is the wordplay.
'beneath' becomes 'under' (I've seen this before).
'english river' becomes 'wear' (river in the north east of England).
'under'+'wear'='UNDERWEAR'
(Other definitions for underwear that I've seen before include "Clothing nearest the skin" , "slips, perhaps" , "Drawers" , "Lingerie" , "shift, perhaps" .)