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' is an anagram of 'IDES'.
'to' acts as a 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" .)