Capital required by old lover (5)
I believe the answer is:
romeo
'lover' is the definition.
(I've seen this before)
'capital required by old' is the wordplay.
'capital' becomes 'rome' (capital of Italy).
'required by' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rome'+'o'='ROMEO'
(Other definitions for romeo that I've seen before include "Shakespearean lover" , "He's a letter read out" , "Comms code for R" , "Code word for the letter R" , "Juliet's immortal lover" .)