Response requiring I send in army unit (7)
I believe the answer is:
riposte
'response' is the definition.
(riposte is a kind of response)
'i send in army unit' is the wordplay.
'send' becomes 'post' (synonyms).
'in' means one lot of letters goes inside another.
'army unit' becomes 're' (Royal Engineers).
'i'+'post'='ipost'
'ipost' placed inside 're' is 'RIPOSTE'.
'requiring' is the link.
(Other definitions for riposte that I've seen before include "Quick retort or return hit in fencing" , "Repartee; quick thrust (fencing)" , "Witty comeback" , "Reply, retort" , "Quick clever reply" .)