Alternate with small hag (6)
I believe the answer is:
switch
'alternate' is the definition.
(I know that alternate can be written as switch)
'with small hag' is the wordplay.
'with' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hag' becomes 'witch' (synonyms).
's' put next to 'witch' is 'SWITCH'.
(Other definitions for switch that I've seen before include "birch" , "Get change" , "Shift" , "Exchange; contactor" , "electrical apparatus" .)