Cheer old troopers during dance (6)
I believe the answer is:
hooray
'cheer' is the definition.
(I've seen this before)
'old troopers during dance' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'troopers' becomes 'or' (military abbreviation for Other Ranks).
'during' means one lot of letters goes inside another.
'dance' becomes 'hay' (type of country dance).
'o'+'or'='oor'
'oor' put within 'hay' is 'HOORAY'.
(Other definitions for hooray that I've seen before include "Yippee!" , "Shout of joy (for Henry?)" , "Exclamation of joy or victory" , "Cheer" , "Hurrah" .)