Fooled about Romeo? Difficult to understand (4)
I believe the answer is:
hard
'difficult to understand' is the definition.
(hard can mean difficult to understand or obscure)
'fooled about romeo?' is the wordplay.
'fooled' becomes 'had' (to have someone can mean to trick them).
'about' means one lot of letters goes inside another.
'romeo?' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'had' going around 'r' is 'HARD'.
(Other definitions for hard that I've seen before include "badly" , "Unyielding" , "Difficult; rigid" , "Solid; tough" , "Firm, rigid, road leading to foreshore" .)