Long house originally used in service (4)
I believe the answer is:
ache
'long' is the definition.
(to long for something is to ache for it)
'house originally used in service' is the wordplay.
'originally' says to take the initial letters.
'used in' indicates putting letters inside.
'service' becomes 'ace' (ace is a kind of service).
The initial letter of 'house' is 'h'.
'h' placed into 'ace' is 'ACHE'.
(Other definitions for ache that I've seen before include "Long ? pain" , "Longing" , "Feel pain" , "Shakespearean character" , "Cause pain" .)