Saw long coat, metre (5)
I believe the answer is:
maxim
'saw' is the definition.
(a saw can mean a proverb or maxim)
'long coat metre' is the wordplay.
'long coat' becomes 'maxi' (I've seen this before).
'metre' becomes 'm' (synonyms).
'maxi'+'m'='MAXIM'
(Other definitions for maxim that I've seen before include "Short pithy statement" , "- Gorky, Russian writer" , "Brief expression of a general principle" , "Pithy statement of a general truth" , "Proverb, aphorism" .)