Daughter nowadays is relative (3)
I believe the answer is:
dad
'is relative' is the definition.
(I've seen this in another clue)
'daughter nowadays' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'd'+'ad'='DAD'
(Other definitions for dad that I've seen before include "Mum's mate" , "Pop, pater" , "Relation" , "Father, in short" , "Popular and informal father" .)