Unit also crossing river, heading for Paris (5)
I believe the answer is:
troop
'unit' is the definition.
(troop is a kind of unit)
'also crossing river heading for paris' is the wordplay.
'also' becomes 'too' ('too' can be a synonym of 'also').
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'heading for' suggests taking the first letters.
The first letter of 'paris' is 'p'.
'too' placed around 'r' is 'troo'.
'troo'+'p'='TROOP'
(Other definitions for troop that I've seen before include "Military group" , "A company, unit" , "Military body" , "A company of people, soldiers or scouts" , "March" .)