In song, the extremes of tenor were quite forceful (6)
I believe the answer is:
strong
'quite forceful' is the definition.
'strong' can be an answer for 'forceful' (similar in meaning). I'm not certain of the 'quite' bit.
'in song the extremes of tenor' is the wordplay.
'in' indicates putting letters inside.
'the extremes of' suggests removing the centre (only the letters at the extremes of the word).
'tenor' with its centre taken out is 'tr'.
'song' enclosing 'tr' is 'STRONG'.
'were' is the link.
Can you help me to learn more?
(Other definitions for strong that I've seen before include "Physically powerful - feel 1 Across" , "Forceful" , "Mighty, powerful" , "Vigorous" , "Powerful; full-flavoured" .)