Discourage explosive reaction initially following clincher (10)
I believe the answer is:
determiner
'clincher' is the definition.
(synonyms)
'discourage explosive reaction initially' is the wordplay.
'discourage' becomes 'deter' (synonyms).
'explosive' becomes 'mine' (e.g. a landmine).
'initially' suggests taking the first letters.
The initial letter of 'reaction' is 'r'.
'deter'+'mine'+'r'='DETERMINER'
'following' is the link.
(Other definitions for determiner that I've seen before include "Perhaps that" , "thing that makes the difference?" , "Settler" .)