Relating to the law of gravitation originally observed in a field near Luxembourg (5)
I believe the answer is:
legal
'relating to the law' is the definition.
'gravitation originally observed in a field near luxembourg' is the wordplay.
'originally' suggests taking the first letters.
'observed in' indicates putting letters inside.
'a field' becomes 'lea' (I've seen this before).
'near' says to put letters next to each other.
'luxembourg' becomes 'l' (international vehicle registration code).
The first letter of 'gravitation' is 'g'.
'g' inserted into 'lea' is 'lega'.
'lega'+'l'='LEGAL'
'of' is the link.
(Other definitions for legal that I've seen before include "Within bounds of law" , "Observing acts" , "sanctioned" , "not breaking the law" , "It's acceptable" .)