It takes days to build a side (4)
I believe the answer is:
ides
'it takes days' is the definition.
The definition suggests a singular noun which matches the answer.
'build a side' is the wordplay.
'build' indicates anagramming the letters.
'side' with letters rearranged gives 'IDES'.
'to' is the link.
Can you help me to learn more?
(Other definitions for ides that I've seen before include "ill-fated time" , "Julius Caesar was told to beware the . . . . of March" , "mid-month for the Romans" , "13th or 15th day of the month in Roman calendar" , "Orfes" .)