Servicemen organise line dancing (12)
I believe the answer is:
legionnaires
'servicemen' is the definition.
(legionnaire is a kind of serviceman)
'organise line dancing' is the wordplay.
'dancing' indicates anagramming the letters.
'organise'+'line'='organiseline'
'organiseline' anagrammed gives 'LEGIONNAIRES'.
(Other definitions for legionnaires that I've seen before include "French soldiers" , "Servicemen" , "troops" .)