Small spaces not good for drains (4)
I believe the answer is:
saps
'drains' is the definition.
(sapping is a kind of draining)
'small spaces not good' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'spaces' becomes 'gaps' (gap is a kind of space).
'not' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'gaps' with 'g' removed is 'aps'.
's'+'aps'='SAPS'
'for' acts as a link.
(Other definitions for saps that I've seen before include "Drains energy from" , "Weakens, deprives of energy" , "Pass around foolish people" , "Weakens, undermines" , "More than one fool" .)