Main issue when hunting is permitted (4,6)
I believe the answer is:
open season
'hunting is permitted' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'main issue' is the wordplay.
'main' becomes 'open sea' (I've seen this before).
'issue' becomes 'son' (issue can mean someone's offspring).
'opensea'+'son'='OPEN SEASON'
'when' is the link.
Can you help me to learn more?
(Other definitions for open season that I've seen before include "When game shootable" , "Hunter's active months" , "period designated for shooting" , "enjoyed by some sportsmen" , "anyone could be opposition here" .)