Rest managed to take writer as a reptile (7)
I believe the answer is:
serpent
'a reptile' is the definition.
(I know that serpent is a type of diapsid)
'rest managed to take writer' is the wordplay.
'managed' indicates anagramming the letters.
'to take' means one lot of letters goes inside another (inserted letters are taken in).
'writer' becomes 'pen' (something which writes).
'rest' is an anagram of 'sert'.
'sert' placed around 'pen' is 'SERPENT'.
'as' acts as a link.
(Other definitions for serpent that I've seen before include "Tempting" , "(In literature) a snake" , "Devil of a snake" , "Instrument of the past" , "Limbless reptile" .)