He's said to be fly (3)
I believe the answer is:
nat
'he's' is the definition.
'said to be fly' is the wordplay.
'said' becomes 't' (I've seen this in other clues).
'to' means one lot of letters go next to another (I've seen this in other clues).
'be fly' becomes 'na' (I can't explain this - if you can you should believe this answer much more).
't' after 'na' is 'NAT'.
Can you help me to learn more?
(Other definitions for nat that I've seen before include "King Cole" , "A nationalist" , "Politician" , "Supporter of the SNP (abbr)" .)