Superb poster plugging new French art portraying river nymphs (7)

'river nymphs' is the definition.
'naiades' can be an answer for 'nymphs' (naiad is a kind of nymph). I'm unsure of the 'river' bit.

'superb poster plugging new french art' is the wordplay.
'superb' becomes 'ai' (I can't justify this - if you can you should believe this answer much more).
'poster' becomes 'ad' (short for advert).
'plugging' is an insertion indicator (inserted letters plug up a gap in the word).
'new' becomes 'n' (abbreviation as in New Testament NT).
'french art' becomes 'es' ('is' in French - 'art' can mean 'is' eg 'thou art').
'ai'+'ad'='aiad'
'n'+'es'='nes'
'aiad' inserted into 'nes' is 'NAIADES'.

'portraying' acts as a link.

Can you help me to learn more?