Awful oil ingested by rat turning up in sewer (6)
I believe the answer is:
tailor
'sewer' is the definition.
(thesaurus)
'awful oil ingested by rat turning up' is the wordplay.
'awful' is an anagram indicator.
'ingested by' is an insertion indicator.
'turning up' says the letters should be written in reverse.
'rat' back-to-front is 'tar'.
'oil' anagrammed gives 'ilo'.
'ilo' placed within 'tar' is 'TAILOR'.
'in' is the link.
(Other definitions for tailor that I've seen before include "person before soldier and sailor in list" , "Maker of men's suits" , "Fashion" , "follower of tinker" , "One who makes garments" .)