We slither having to wriggle past (9)
I believe the answer is:
erstwhile
'past' is the definition.
(similar in meaning)
'we slither having to wriggle' is the wordplay.
'having to wriggle' is an anagram indicator.
'we'+'slither'='weslither'
'weslither' with letters rearranged gives 'ERSTWHILE'.
(Other definitions for erstwhile that I've seen before include "Former, previous" , "Quondam" , "In the past" , "Formerly" , "Former - list where (anag)" .)