Combined action (7)
I believe the answer is:
synergy
'combined action' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for synergy that I've seen before include "Cooperative interaction" , "Productive cooperation" , "Mutual cooperation producing greater effect" , "Effect where combined actions produce a greater effect than the sum of individual efforts" , "Combined action" .)