Barrier built by day, in the morning (3)
I believe the answer is:
dam
'barrier' is the definition.
(dam is a kind of barrier)
'day in the morning' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'in the morning' becomes 'am' (AM on a clock).
'd'+'am'='DAM'
'built by' is the link.
(Other definitions for dam that I've seen before include "Weir" , "A mad sort of mum" , "Mother of quadruped" , "Animal's mother" , "Block (river)" .)