Muscle ailing during army corps dance (9)
I believe the answer is:
quadrille
'dance' is the definition.
(quadrille is a kind of dance)
'muscle ailing during army corps' is the wordplay.
'muscle' becomes 'quad' (quad is a kind of muscle).
'ailing' becomes 'ill' (similar in meaning).
'during' is an insertion indicator.
'army corps' becomes 'RE' (Royal Engineers).
'ill' going within 're' is 'rille'.
'quad'+'rille'='QUADRILLE'
(Other definitions for quadrille that I've seen before include "(Music for a) square dance for four or more couples" , "Old square dance (performed by lobsters in Alice in Wonderland)" .)