Prompt treatment for fits out West (5)
I believe the answer is:
swift
'prompt' is the definition.
(I've seen this before)
'treatment for fits out west' is the wordplay.
'treatment for' indicates an anagram (I've seen this in other clues).
'out' is an insertion indicator.
'west' becomes 'W' (abbreviation).
'fits' is an anagram of 'sift'.
'sift' placed around 'w' is 'SWIFT'.
(Other definitions for swift that I've seen before include "Jonathan . . . . . wrote 'Gulliver's Travels'" , "Swallow-like bird" , "like a cheetah?" , "Racy" , "Quick, prompt" .)