Female swan protected by one, as so flustered when hunting is allowed (4,6)
I believe the answer is:
open season
'when hunting is allowed' is the definition.
(period when hunting is permitted)
'female swan protected by one as so flustered' is the wordplay.
'female swan' becomes 'pen' (I've seen this before).
'protected by' means one lot of letters goes inside another.
'flustered' indicates anagramming the letters.
'one'+'as'+'so'='oneasso'
'oneasso' with letters rearranged gives 'oseason'.
'pen' put within 'oseason' is 'OPEN SEASON'.
(Other definitions for open season that I've seen before include "when hunting rules are relaxed" , "Free time for game" , "When game shootable" , "anyone could be opposition here" , "period designated for shooting" .)