Dad's oldish doctor at heart is a chronic user (6)
I believe the answer is:
addict
'chronic user' is the definition.
'addict' can be an answer for 'user' (I've seen this before). I'm not sure about the 'chronic' bit.
'dad's oldish doctor at heart' is the wordplay.
'dad's oldish' becomes 'addi' (I can't explain this - if you can you should give a lot more credence to this answer).
'at heart' means to look at the middle letters.
The central letters of 'doctor' are 'ct'.
'addi'+'ct'='ADDICT'
'is a' is the link.
Can you help me to learn more?
(Other definitions for addict that I've seen before include "fiend" , "One who's hooked" , "Slave to a chemical substance" , "Someone who's hooked?" , "One hooked on something" .)