A girl is after small steam room (5)
I believe the answer is:
sauna
'steam room' is the definition.
(I know that sauna is a type of steam room)
'a girl is after small' is the wordplay.
'girl' becomes 'una' (girl's name).
'is after' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'una'='auna'
'auna' after 's' is 'SAUNA'.
(Other definitions for sauna that I've seen before include "Type of bath" , "Where one is sweaty" , "Finnish steam bath" , "Finnish-style bath" , "A Scandinavian steam bath" .)