Choose one key to drinks dispenser (5)
I believe the answer is:
optic
'drinks dispenser' is the definition.
(I've seen this before)
'choose one key' is the wordplay.
'choose' becomes 'opt' (both can mean to pick something).
'one' becomes 'i' (Roman numeral).
'key' becomes 'c' (musical key).
'opt'+'i'+'c'='OPTIC'
'to' is the link.
(Other definitions for optic that I've seen before include "Of sight or the eye" , "Body part" , "of a viewer?" , "Device for measuring out spirits" , "Pertaining to the eye" .)