Drain odd parts of swamp (3)
I believe the answer is:
sap
'drain' is the definition.
(sap can mean to suck or drain)
'odd parts of swamp' is the wordplay.
'odd parts of' indicates alternate letters (letters in odd-numbered positions).
The alternating letters of 'swamp' are 'SAP'.
(Other definitions for sap that I've seen before include "Vital plant juice" , "Weak foolish person" , "Cosh; plant's juice" , "Watery substance in a plant" , "Enervate" .)