Crawler, irritating little thing in failing cult (5,6)
I believe the answer is:
stick insect
'crawler' is the definition.
(insect which crawls)
'irritating little thing in failing cult' is the wordplay.
'irritating little thing' becomes 'tick' (irritating parasite).
'in' means one lot of letters goes inside another.
'failing' becomes 'sin' (I've seen this in another clue).
'cult' becomes 'sect' (religious sect).
'tick' inserted within 'sin' is 'stickin'.
'stickin'+'sect'='STICK-INSECT'
(Other definitions for stick insect that I've seen before include "One well-camouflaged" , "It resembles a twig" , "Twiggy type" , "Twiggy lookalike" , "Thin person" .)