Long drain beginning to dry out (4)
I believe the answer is:
itch
'long drain' is the definition.
'itch' can be an answer for 'long' (to long or itch for something is to desire it). I am unsure of the 'drain' bit.
'beginning to dry out' is the wordplay.
'beginning' says to take the initial letters.
'dry' becomes 'ich' (I can't explain this - if you can you should give a lot more credence to this answer).
'out' indicates putting letters inside.
The initial letter of 'to' is 't'.
't' placed inside 'ich' is 'ITCH'.
(Other definitions for itch that I've seen before include "Teasing desire" , "minor irritant" , "cause irritation" , "Rash reaction?" , "Contagious skin infection" .)