Smart police force once occupying small gym (6)
I believe the answer is:
spruce
'smart' is the definition.
('spruce' can be a synonym of 'smart')
'police force once occupying small gym' is the wordplay.
'police force once' becomes 'ruc' (Royal Ulster Constabulary).
'occupying' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gym' becomes 'pe'.
's'+'pe'='spe'
'ruc' going within 'spe' is 'SPRUCE'.
(Other definitions for spruce that I've seen before include "Well-groomed - tree" , "wood" , "Smart in appearance like fir tree" , "thing for fairy to be on top of?" , "Well-dressed" .)