Different topless parent (5)
I believe the answer is:
other
'different' is the definition.
(similar in meaning)
'topless parent' is the wordplay.
'topless' means to remove the first letter.
'parent' becomes 'mother' (type of parent).
'mother' with its initial letter removed is 'OTHER'.
(Other definitions for other that I've seen before include "Different; additional" , "Second" , "One may be significant" , "Alternative - unrelated" , "Different, disparate" .)