You shouldn't have relations over, they're a burden every year (5)
I believe the answer is:
taxes
'a burden every year' is the definition.
I can't judge whether this definition defines the answer.
'you shouldn't have relations over' is the wordplay.
'you shouldn't have' becomes 'ta' (both can mean 'thank you').
'relations' becomes 'sex' (euphemism for sex).
'over' is a reversal indicator.
'sex' reversed gives 'xes'.
'ta'+'xes'='TAXES'
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for taxes that I've seen before include "Levies imposed by government" , "Levies duty" , "Levies, strains" , "Puts great strain on, as Revenue Commissioners do" .)