Parent loses first alternative (5)
I believe the answer is:
other
'alternative' is the definition.
(I've seen this in another clue)
'parent loses first' is the wordplay.
'parent' becomes 'mother' (mother is a kind of parent).
'loses first' means to remove the first letter (I've seen 'losing first' mean this).
'mother' with its initial letter taken away is 'OTHER'.
(Other definitions for other that I've seen before include "Remaining" , "else!" , "Alternative - unrelated" , "Different (from those mentioned)" , "Different, alternative" .)