Beauty in Romeo overcome by small-minded behaviour (10)
I believe the answer is:
prettiness
'beauty' is the definition.
(I know that prettiness is a type of beauty)
'romeo overcome by small-minded behaviour' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'overcome by' is an insertion indicator.
'small-minded behaviour' becomes 'pettiness' (I've seen this before).
'r' going within 'pettiness' is 'PRETTINESS'.
'in' acts as a link.
(Other definitions for prettiness that I've seen before include "13 [ATTRACTIVE] state" , "Attractive quality" , "1 across (anag)" .)