See a measure as a sure thing (5)
I believe the answer is:
cinch
'a sure thing' is the definition.
(I've seen this in another clue)
'see a measure' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'a measure' becomes 'inch' (inch is a unit of length).
'c'+'inch'='CINCH'
'as' is the link.
(Other definitions for cinch that I've seen before include "simple job" , "Doddle" , "A walk in the park" , "Something easily done" , "piece of cake" .)