Romeo in theatrical muscle pain (5)
I believe the answer is:
cramp
'muscle pain' is the definition.
(I've seen this before)
'romeo in theatrical' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'theatrical' becomes 'camp' (I've seen this before).
'r' inserted into 'camp' is 'CRAMP'.
(Other definitions for cramp that I've seen before include "hamper" , "Contract" , "writers may be familiar with" , "Muscular spasm, very painful" , "Severe muscular pain" .)