Sounded thrilled: and so to the end, off and on (5)
I believe the answer is:
oohed
'sounded thrilled' is the definition.
(I've seen this before)
'so to the end off and on' is the wordplay.
'off and on' indicates alternate letters (letters used at regular intervals).
'so'+'to'+'the'+'end'='sototheend'
The alternate letters of 'sototheend' are 'OOHED'.
'and' acts as a link.
(Other definitions for oohed that I've seen before include "showed pleasure" , "Sounded thrilled" .)