Alternate with small hag (6)
I believe the answer is:
switch
'alternate' is the definition.
(synonyms)
'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'+'witch' is 'SWITCH'.
(Other definitions for switch that I've seen before include "birch" , "Get change" , "Shift" , "Exchange; contactor" , "electrical apparatus" .)