Conspirator against Caesar, we hear, heartless guy in combined action (7)
I believe the answer is:
synergy
'combined action' is the definition.
(I've seen this before)
'conspirator against caesar we hear heartless guy' is the wordplay.
'conspirator against caesar' becomes 'cinna' (Cinna was a conspirator against Julius Caesar).
'we hear' indicates a 'sounds like' (homophone) clue.
'heartless' suggests removing the centre.
'guy' with its middle removed is 'gy'.
'cinna' sounds like 'syner'.
'syner'+'gy'='SYNERGY'
'in' acts as a link.
(Other definitions for synergy that I've seen before include "Beneficial interaction" , "Coordinated action" , "Extra benefit of working together" , "Productive cooperation" , "Working together to greater effect" .)