Nymph gently donning lacy pants thus (7)
I believe the answer is:
calypso
'nymph' is the definition.
(I know that Calypso is a type of sea nymph)
'gently donning lacy pants thus' is the wordplay.
'gently' becomes 'p' (musical abbreviation for 'piano', softly).
'donning' indicates putting letters inside (I've seen this in other clues).
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'thus' becomes 'so' (synonyms).
'lacy' with letters rearranged gives 'caly'.
'caly'+'so'='calyso'
'p' going into 'calyso' is 'CALYPSO'.
(Other definitions for calypso that I've seen before include "West Indian music style" , "West Indian music - nymph who kept Odysseus on her island" , "Gk. nymph; Caribbean ballad" , "Topical West Indian song - orchid - sea nymph" , "Ballad - dance - orchid - sea nymph - moon of Saturn" .)