Graphical user interface on sailor's instruments (7)
I believe the answer is:
guitars
'instruments' is the definition.
(guitar is a kind of instrument)
'graphical user interface on sailor's' is the wordplay.
'graphical user interface' becomes 'gui' (this might be a standard abbreviation which I don't know about).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'sailor's' becomes 'tar's' (slang term for a sailor).
'gui'+'tars'='GUITARS'
Can you help me to learn more?
(Other definitions for guitars that I've seen before include "Stringed instruments" , "Axes" .)