Denied that I sang out “Help!” (8)
I believe the answer is:
gainsaid
'denied' is the definition.
(I've seen this before)
'i sang out help' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'help' becomes 'aid' ('aid' can be a synonym of 'help'**).
'i'+'sang'='isang'
'isang' with letters rearranged gives 'gains'.
'gains'+'aid'='GAINSAID'
'that' is the link.
(Other definitions for gainsaid that I've seen before include "Formally denied or contradicted" , "Challenged" .)