Siesta after one — and time is not appropriate (5)
I believe the answer is:
inapt
'not appropriate' is the definition.
(inapt means inappropriate or ill-fitting)
'siesta after one and time' is the wordplay.
'siesta' becomes 'nap' (I've seen this before).
'after' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'and' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'nap' after 'i' is 'inap'.
'inap'+'t'='INAPT'
'is' is the link.
(Other definitions for inapt that I've seen before include "Ill-chosen, not suitable" , "It's clumsy" , "Inappropriate" , "not seemly" , "Improper" .)