Spots small containers keeping ale's head (6)
I believe the answer is:
stains
'spots' is the definition.
(spot can mean a stain or mark)
'small containers keeping ale's head' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'containers' becomes 'tins' (tin is a kind of container).
'keeping' is an insertion indicator.
'head' indicates taking the first letters.
The initial letter of 'ales' is 'a'.
'tins' placed around 'a' is 'tains'.
's'+'tains'='STAINS'
(Other definitions for stains that I've seen before include "Dirty marks" , "Black marks for saints" , "Discolorations on satins" , "Discolours or dyes wood" , "Sullies" .)