Young man with an old heart nowadays (3)
I believe the answer is:
lad
'young man' is the definition.
(I've seen this before)
'with an old heart nowadays' is the wordplay.
'with' says to put letters next to each other.
'heart' means to look at the middle letters.
'nowadays' becomes 'ad'.
The middle letter of 'old' is 'l'.
'l' put next to 'ad' is 'LAD'.
(Other definitions for lad that I've seen before include "Boy, young man" , "Youngster" , "Boy, familiar" , "Stable employee" , "Chap, fellow" .)