Means of identification of crop, we hear, more affected by cold? (6,6)
I believe the answer is:
serial number
'means of identification' is the definition.
(a serial number identifies something)
'crop we hear more affected by cold?' is the wordplay.
'crop' becomes 'cereal' (cereal is a crop).
'we hear' indicates a 'sounds like' (homophone) clue.
'more affected by cold?' becomes 'number' (more numb with cold).
'cereal' sounds like 'serial'.
'serial'+'number'='SERIAL NUMBER'
'of' is the link.
(Other definitions for serial number that I've seen before include "Product identifier" , "eg Eastenders tune" , "may be seen on your TV" , "It may identify one of a batch of items" , "One can specifically identify" .)