Each day before run, start to yawn (5)
I believe the answer is:
every
'each' is the definition.
(synonyms)
'day before run start to yawn' is the wordplay.
'day before' becomes 'eve' (the eve of something is the night before).
'run' becomes 'r' (cricket abbreviation).
'start to' suggests taking the first letters.
The initial letter of 'yawn' is 'y'.
'eve'+'r'+'y'='EVERY'
(Other definitions for every that I've seen before include "Each single (item)" , "Each one without exception" , "All possible; the utmost" , "'God bless us - - one !'" , "As much as possible" .)