Disappointed, but never by a supporter! (3,4)

I believe the answer is:

let down

I'm a little stuck... Click here to teach me more about this clue!

'disappointed' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for let down that I've seen before include "Failed - to deflate a tyre - disappointed" , "Aircraft descent" , "Disappoint, betray" , "Disappoint, fail to keep promise" , "Lower" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app