Many a measure regarded as a sure thing (5)
I believe the answer is:
cinch
'a sure thing' is the definition.
(I've seen this in another clue)
'many a measure' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for i00).
'a measure' becomes 'inch' (inch is a unit of length).
'c'+'inch'='CINCH'
'regarded as' is the link.
(Other definitions for cinch that I've seen before include "In slang, a certainty or easy task" , "simple job" , "Doddle" , "Something easily done" , "piece of cake" .)