Have to pay for when not there, oddly (3)
I believe the answer is:
owe
'have to pay' is the definition.
(I've seen this in another clue)
'for when not there oddly' is the wordplay.
'not there oddly' means one should take alternating letters (letters in odd-numbered positions removed).
'for'+'when'='forwhen'
The alternating letters of 'forwhen' are 'OWE'.
(Other definitions for owe that I've seen before include "have favours to return" , "Be in debt - woe is me" , "Have outstanding" , "Are debtors" , "Have a duty to pay" .)