Different relative loses head (5)
I believe the answer is:
other
'different' is the definition.
(similar in meaning)
'relative loses head' is the wordplay.
'relative' becomes 'mother' (mother is a kind of relative).
'loses head' means to remove the first letter.
'mother' with its first letter taken away is 'OTHER'.
(Other definitions for other that I've seen before include "Different, not the same one or this one" , "Remaining" , "Different, alternative" , "Very unusual" , "Alternative of two" .)