Determined to catch grey cuckoo, immediately before heron (5,5)

'heron' is the definition.
(snowy egret is a kind of heron)

'determined to catch grey cuckoo immediately before' is the wordplay.
'determined' becomes 'set' (similar in meaning).
'to catch' means one lot of letters goes inside another.
'cuckoo' indicates anagramming the letters.
'immediately' becomes 'now' (I've seen this in another clue).
'before' means one lot of letters go next to another.
'grey' is an anagram of 'yegr'.
'yegr' after 'now' is 'nowyegr'.
'set' going around 'nowyegr' is 'SNOWY EGRET'.