Cross is metre longer perhaps (7)
I believe the answer is:
mongrel
'cross' is the definition.
(I've seen this before)
'metre longer perhaps' is the wordplay.
'metre' becomes 'm' (synonyms).
'perhaps' indicates anagramming the letters.
'longer' with letters rearranged gives 'ongrel'.
'm'+'ongrel'='MONGREL'
'is' acts as a link.
(Other definitions for mongrel that I've seen before include "of unknown variety" , "Hybrid" , "mixed stock" , "Cross" , "Cur" .)