Nag Romeo, in tights (5)
I believe the answer is:
horse
'nag' is the definition.
(a nag is an old horse)
'romeo in tights' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'tights' becomes 'hose' (synonyms).
'r' put inside 'hose' is 'HORSE'.
(Other definitions for horse that I've seen before include "Quadruped; cavalry" , "Domesticated animal" , "Drier" , "Mount" , "Shore (anag)" .)