After straying in, fancy that large swain? (9)
I believe the answer is:
waterfowl
'swain?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'after straying in fancy that large' is the wordplay.
'straying' is an anagram indicator.
'in' is an insertion indicator.
'fancy that' becomes 'wow' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'large' becomes 'L' (eg in clothes sizes).
'after' with letters rearranged gives 'aterf'.
'aterf' inserted within 'wow' is 'waterfow'.
'waterfow'+'l'='WATERFOWL'
Can you help me to learn more?
(Another definition for waterfowl that I've seen is " Aquatic game birds".)