Strident Welshwoman stifles singer keeping time (10)
I believe the answer is:
stentorian
'strident' is the definition.
(thesaurus)
'welshwoman stifles singer keeping time' is the wordplay.
'welshwoman' becomes 'sian' (common Welsh name).
'stifles' is an insertion indicator.
'singer' becomes 'tenor' (tenor is a kind of singer**).
'keeping' means one lot of letters goes inside another.
'time' becomes 't'.
'tenor' placed around 't' is 'tentor'.
'sian' going around 'tentor' is 'STENTORIAN'.
(Other definitions for stentorian that I've seen before include "booming" , "Strident" , "(Of a person's voice) loud and powerful" .)