Joined force, displaying prior employment (5)
I believe the answer is:
fused
'joined' is the definition.
(fused means joined together)
'force displaying prior employment' is the wordplay.
I cannot really understand how this works, but
'force' could be 'f' (abbreviation in physics e.g. f=ma) and 'f' is found within the answer.
'employment' could be 'used' (I've seen this before) and 'used' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fused that I've seen before include "Married" , "Welded together by heat" , "Joined by melting" , "Stuck together" , "Coalesced" .)