Squirm beside sweetheart embracing Romeo (6)
I believe the answer is:
writhe
'squirm' is the definition.
(synonyms)
'beside sweetheart embracing romeo' is the wordplay.
'beside' becomes 'with' (I've seen this in another clue).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'embracing' indicates putting letters inside.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'with'+'e'='withe'
'withe' going around 'r' is 'WRITHE'.
(Other definitions for writhe that I've seen before include "Twist in agony" , "Contort in pain" , "Thrash" , "Twist as in pain" , "6, maybe in agony" .)