Came in, running small theatres (7)
I believe the answer is:
cinemas
'theatres' is the definition.
(cinema is a kind of theatre)
'came in running small' is the wordplay.
'running' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'came'+'in'='camein'
'camein' is an anagram of 'cinema'.
'cinema'+'s'='CINEMAS'
(Other definitions for cinemas that I've seen before include "Where films are shown" , "Picture-houses" .)