To break a strike is a tremendous success (5,3)
I believe the answer is:
smash hit
'tremendous success' is the definition.
'smash hit' can be an answer for 'success' (I have seen 'Great success' mean 'smash hit' so perhaps 'success' could also mean 'smash hit'). I'm not certain of the 'tremendous' bit.
'to break a strike' is the wordplay.
'to break' becomes 'smash' (smashing is a kind of breaking).
'a strike' becomes 'hit' ('hit' can be a synonym of 'strike').
'smash'+'hit'='SMASH-HIT'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for smash hit that I've seen before include "blockbuster" , "number one" , "Great success" , "great entertainment" , "Huge success" .)