Reptiles coming from opposite directions -- fishes losing head (6)
I believe the answer is:
snakes
'reptiles' is the definition.
(snake is a kind of reptile)
'opposite directions fishes losing head' is the wordplay.
'opposite directions' becomes 'sn' (opposite compass points).
'fishes' becomes 'hakes' (hake is a kind of fish).
'losing head' means to remove the first letter.
'hakes' with its initial letter taken away is 'akes'.
'sn'+'akes'='SNAKES'
'coming from' acts as a link.
(Other definitions for snakes that I've seen before include "Reptiles, some venomous" , "Winds; adders" , "Limbless reptiles" , "Go down them on board" , "Adders, cobras, rattlers etc" .)