Brother in English sea (5)
I believe the answer is:
brine
'sea' is the definition.
(I've seen this in other clues)
'brother in english' is the wordplay.
'brother' becomes 'br' (abbreviation in titles of monks).
'english' becomes 'e' (abbreviation).
'br'+'in'+'e'='BRINE'
(Other definitions for brine that I've seen before include "preservative" , "Salted water" , "Salty water" , "Solution of salt in water" , "Sea water" .)