Daughter nowadays is a relative (3)
I believe the answer is:
dad
'relative' is the definition.
(I know that dad is a type of fathers)
'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'
'is a' is the link.
(Other definitions for dad that I've seen before include "Father, in short" , "Papa" , "Popular father - a palindrome" , "Pa" , "Relation" .)