Stream runs through reserve (5)
I believe the answer is:
brook
'stream' is the definition.
(I know that brook is a type of stream)
'runs through reserve' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'reserve' becomes 'book' (to book something is to reserve it).
'r' put into 'book' is 'BROOK'.
(Other definitions for brook that I've seen before include "stomach" , "Bear - small stream" , "Stand for" , "Small fresh-water stream" , "running water" .)