Stamina required to cross river or brook (6)
I believe the answer is:
stream
'brook' is the definition.
(I've seen this before)
'stamina required to cross river' is the wordplay.
'stamina' becomes 'steam' (to run out of steam is to run out of stamina).
'required to cross' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'steam' placed around 'r' is 'STREAM'.
'or' is the link.
(Other definitions for stream that I've seen before include "course" , "River - current" , "Master (anag) - current" , "Flowing water" , "divide pupils into ability groups" .)