Engulf wobbly bum in woollen fabric (8)
I believe the answer is:
submerge
'engulf' is the definition.
(thesaurus)
'wobbly bum in woollen fabric' is the wordplay.
'wobbly' is an anagram indicator (I've seen this in other clues).
'in' is an insertion indicator.
'woollen fabric' becomes 'serge' (serge is a type of fabric).
'bum' is an anagram of 'ubm'.
'ubm' placed within 'serge' is 'SUBMERGE'.
(Other definitions for submerge that I've seen before include "Cover" , "Engulf, immerse" , "Go under water" , "Disappear below surface" , "swill round" .)