Former band, say, bang on (5)
I believe the answer is:
exact
'bang on' is the definition.
(bang on means exactly right)
'former band say' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'band say' becomes 'act' (band is a kind of act).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "far from improvised?" , "Correct; insist on" , "on the money" , "Precise; extort" , "Force out" .)