Reason gendarmes initially seen in a street in Paris (5)
I believe the answer is:
argue
'reason' is the definition.
(I know that reason can be written as argue)
'gendarmes initially seen in a street in paris' is the wordplay.
'initially' indicates taking the first letters.
'seen in' indicates putting letters inside.
'street in paris' becomes 'rue' ('street' in French).
The first letter of 'gendarmes' is 'g'.
'a'+'rue'='arue'
'g' inserted inside 'arue' is 'ARGUE'.
(Other definitions for argue that I've seen before include "Present the case for" , "Contend with reasoning" , "Debate, contend" , "Debate, row" , "Debate, contest verbally" .)