Diana on instrument to reduce concentration (6)
I believe the answer is:
dilute
'reduce concentration' is the definition.
(I've seen this before)
'diana on instrument' is the wordplay.
'diana' becomes 'Di' (Di is short for Diana).
'on' says to put letters next to each other.
'instrument' becomes 'lute' (lute is a kind of instrument).
'di'+'lute'='DILUTE'
'to' is the link.
(Other definitions for dilute that I've seen before include "lost concentration" , "Tone down" , "Make (a liquid) thinner" , "having less concentration" , "Add liquid to lessen strength" .)