Swamp, however nasty, looked magical initially (9)
I believe the answer is:
overwhelm
'swamp' is the definition.
(thesaurus)
'however nasty looked magical initially' is the wordplay.
'nasty' indicates anagramming the letters.
'initially' suggests taking the first letters.
The initial letters of 'looked magical' is 'lm'.
'however' is an anagram of 'overwhe'.
'overwhe'+'lm'='OVERWHELM'
(Other definitions for overwhelm that I've seen before include "Too much" , "Inundate, engulf" , "Overcome by superior force" , "Submerge" , "Bury or drown beneath a huge mass of something" .)