Many a measure is a certainty (5)
I believe the answer is:
cinch
'a certainty' is the definition.
(I've seen this before)
'many a measure' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for i00).
'a measure' becomes 'inch' (inch is a kind of measure).
'c'+'inch'='CINCH'
'is' is the link.
(Other definitions for cinch that I've seen before include "Doddle" , "A walk in the park" , "In slang, a certainty or easy task" , "simple job" , "Something easily done" .)