Under no obligation to observe the customs? (4-4)
I believe the answer is:
duty-free
'under no' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'obligation to observe the customs?' is the wordplay.
'obligation' becomes 'duty' (synonyms).
'to observe the customs?' becomes 'free' (I am not sure about this - if you are sure you should believe this answer much more).
'duty'+'free'='DUTY-FREE'
Can you help me to learn more?
(Other definitions for duty-free that I've seen before include "Not subject to tax, as at airports" , "No tax payable as in airports" , "With exemption from customs tax" , "without obligation?" , "Describing of a kind of airport shop" .)