Vital action in pool for typist's depression? (9)
I believe the answer is:
keystroke
'typist's depression?' is the definition.
(a typist pressing or depressing a key)
'vital action in pool' is the wordplay.
'vital' becomes 'key' (I've seen this in other clues).
'action in pool' becomes 'stroke' (swimming stroke).
'key'+'stroke'='KEYSTROKE'
'for' acts as a link.
(Other definitions for keystroke that I've seen before include "Single motion during typing" , "One character typed" , "Single operation on a typewriter" , "Action that's pressing" , "One tap" .)