Setting up base on time is an obligation (4)
I believe the answer is:
debt
'an obligation' is the definition.
('debt' can be a synonym of 'obligation')
'setting up base on time' is the wordplay.
'setting up' says the letters should be written in reverse.
'base' becomes 'bed' (bed is a kind of base**).
'on' says to put letters next to each other.
'time' becomes 't'.
'bed' reversed gives 'deb'.
'deb'+'t'='DEBT'
'is' is the link.
(Other definitions for debt that I've seen before include "Money owed or due" , "Commitment" , "Something owed" , "Financial problem?" , "Financial liability" .)