Nymph in rock poster? (5)
I believe the answer is:
oread
'nymph' is the definition.
(I know that Oread is a type of Greek nymph)
'rock poster?' is the wordplay.
'rock' becomes 'ore' (ores are found in rocks).
'poster?' becomes 'ad' (short for advert).
'ore'+'ad'='OREAD'
'in' acts as a link.
(Other definitions for oread that I've seen before include "Maid of the Mountains" , "O dear, a strange mountain nymph" , "Natural spirit" , "mountainous girl" , "spirit at peak?" .)