Divine ruby stolen by ancient Briton (7)
I believe the answer is:
predict
'divine' is the definition.
(I've seen this before)
'ruby stolen by ancient briton' is the wordplay.
'ruby' becomes 'red' (synonyms).
'stolen by' is an insertion indicator.
'ancient briton' becomes 'pict' (I've seen this before).
'red' inserted into 'pict' is 'PREDICT'.
(Other definitions for predict that I've seen before include "Forecast" , "Foretell, prophesy" , "Estimate consequence of" , "Anticipate" , "Divine" .)