I went off river when it was cold (6)
I believe the answer is:
winter
'it was cold' is the definition.
The definition suggests a singular noun which matches the answer.
'i went off river' is the wordplay.
'off' indicates anagramming the letters.
'river' becomes 'r' (abbreviation for river).
'i'+'went'='iwent'
'iwent' is an anagram of 'winte'.
'winte'+'r'='WINTER'
'when' is the link.
Can you help me to learn more?
(Other definitions for winter that I've seen before include "Cold season" , "Cold period" , "Time for mittens and scarves?" , "'Now is the . . . . . . of our discontent' (Richard III)" , "'If . . . . . . comes, can spring be far behind?'" .)