Lanky guy at first catching fish (8)
I believe the answer is:
gangling
'lanky' is the definition.
('gangling' can be a synonym of 'lanky')
'guy at first catching fish' is the wordplay.
'at first' suggests taking the first letters.
'catching fish' becomes 'angling' (angling is a kind of catching fish).
The first letter of 'guy' is 'g'.
'g'+'angling'='GANGLING'
(Other definitions for gangling that I've seen before include "moving awkwardly" , "Tall and thin" , "Rawboned" , "Lanky" .)