I am joining alliance in strike (6)
I believe the answer is:
impact
'strike' is the definition.
(I know that strike is a more specific form of the action impact)
'i am joining alliance' is the wordplay.
'i am' becomes 'I'm' (contraction).
'joining' is a charade indicator (letters next to each other).
'alliance' becomes 'pact' (I've seen this in another clue).
'im'+'pact'='IMPACT'
'in' acts as a link.
(Other definitions for impact that I've seen before include "Force exerted by collision" , "Serious effect" , "hit, in a big way" , "Forceful consequence" , "Repercussions" .)