Increase power, perhaps, crossing river against the current (8)
I believe the answer is:
upstream
'against the current' is the definition.
(synonyms)
'increase power perhaps crossing river' is the wordplay.
'increase' becomes 'up' (upping is a kind of increasing).
'power perhaps' becomes 'steam' (steam power).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'steam' enclosing 'r' is 'stream'.
'up'+'stream'='UPSTREAM'
(Other definitions for upstream that I've seen before include "at an earlier stage in process" , "contraflow" , "Away from the sea" , "Ma erupts (anag.)" , "Facing current" .)