Series of hostile actions friendly? Not entirely! (3)
I believe the answer is:
war
'series of hostile actions' is the definition.
'war' can be an answer for 'series' (I have seen 'series of engagements' mean 'war' so perhaps 'series' could also mean 'war'). I'm not certain of the 'of hostile actions' bit.
'friendly? not entirely' is the wordplay.
'friendly?' becomes 'warm' (synonym).
'not entirely' means to remove the last letter (most but not all of the word).
'warm' with its last letter taken off is 'WAR'.
Can you help me to learn more?
(Other definitions for war that I've seen before include "Strife" , "Combat" , "Period of hostilities" , "Fight" , "Major conflict" .)