Swamp, however nasty, looked magical initially (9)
I believe the answer is:
Here is my best explanation:
'swamp' is the definition.
'however nasty looked magical initially' is the subsidiary indication.
'nasty' indicates an anagram.
'initially' says to take the initial letters.
The initial letters of 'looked magical' is 'lm'.
'however' is an anagram of 'overwhe'.
(Other definitions for overwhelm that I've seen before include "Crush", "swamp", "Submerge", "Bury or drown beneath a huge mass of something", "Overcome by superior force", "Completely defeat", "Snow under", "Inundate, engulf", "Conquer" and "Engulf".)