File snake under heading for reptiles (4)
I believe the answer is:
rasp
'file' is the definition.
(rasp is a kind of file)
'snake under heading for reptiles' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'under' says to put letters next to each other.
'heading for' suggests taking the first letters.
The first letter of 'reptiles' is 'r'.
'asp' put after 'r' is 'RASP'.
(Other definitions for rasp that I've seen before include "Harsh grating sound" , "fruit" , "Abrade" , "unpleasant noise" , "Grate; scrape" .)