Second postman in a row (4)
I believe the answer is:
spat
'a row' is the definition.
(a spat is a disagreement or argument)
'second postman' is the wordplay.
'second' becomes 's' (common abbreviation).
'postman' becomes 'pat' (children's TV character Postman Pat).
's'+'pat'='SPAT'
'in' acts as a link.
(Other definitions for spat that I've seen before include "Minor argument" , "Quarrel; oyster spawn" , "Slight argument" , "Gestured with contempt" , "Disagreement" .)