Supplier of distilleries certainly backing installing power (4)
I believe the answer is:
spey
'supplier' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'distilleries certainly backing installing power' is the wordplay.
'backing' suggests the final letters.
'installing' indicates putting letters inside (inserted letters are installed).
'power' becomes 'pe' (P.E. is an example).
The last letters of 'distilleries certainly' are 'sy'.
'sy' enclosing 'pe' is 'SPEY'.
'of' is the link.
Can you help me to learn more?
(Other definitions for spey that I've seen before include "Salmon fishing river flowing into the Moray Firth" , "Scottish river, important for salmon fishing and whisky" , "Aviemore river" .)