At heart love, I must think, is a generational thing (4)
I believe the answer is:
ovum
'a generational thing' is the definition.
(egg cell used to make a new human)
'at heart love i must think' is the wordplay.
'at heart' indicates the central letters.
'i must think' becomes 'um' (expression of hesitation).
The central letters of 'love' are 'ov'.
'ov'+'um'='OVUM'
'is' acts as a link.
(Other definitions for ovum that I've seen before include "Reproductive cell, 'egg' in Latin" , "bit of life potentially" , "Human egg cell" , "Female reproductive cell" , "A female egg-cell" .)