Underworld takes care of swells (8)
I believe the answer is:
distends
'swells' is the definition.
('distend' can be a synonym of 'swell')
'underworld takes care of' is the wordplay.
'underworld' becomes 'dis' (I've seen this before).
'takes care of' becomes 'tends' (tending is a kind of taking care of).
'dis'+'tends'='DISTENDS'
(Other definitions for distends that I've seen before include "Becomes wider" , "stretches out" , "bulges" , "Swells by pressure from within" , "Expands, dilates" .)