Regular payments to secure Arab stringed instrument (4)
I believe the answer is:
harp
'stringed instrument' is the definition.
(harp is a kind of stringed instrument)
'regular payments to secure arab' is the wordplay.
'regular payments' becomes 'hp' (hire purchase, payment made in instalments).
'to secure' indicates putting letters inside.
'arab' becomes 'ar' (abbreviation in reference works).
'hp' enclosing 'ar' is 'HARP'.
(Other definitions for harp that I've seen before include "Musical symbol" , "Large plucked instrument" , "''The ... that once through Tara's halls'' (4)" , "Stringed instrument" , "Talk tiresomely using upright musical instrument" .)