Spoil most of river, clogged off and on (7)
I believe the answer is:
indulge
'spoil' is the definition.
(I know that spoil can be written as indulge)
'most of river clogged off and on' is the wordplay.
'most of' means to remove the last letter.
'river' becomes 'indus' (Indian river).
'off and on' indicates alternate letters (letters taken on and off, on a regular basis).
The alternating letters of 'clogged' are 'lge'.
'indus' with its last letter taken off is 'indu'.
'indu'+'lge'='INDULGE'
(Other definitions for indulge that I've seen before include "Allow person to have what he/she wishes" , "Coddle while eluding." , "Gratify, spoil" , "Allow (someone) to enjoy a pleasure" , "Favour, gratify" .)