Head but no heart when he's in charge of restaurant (4)
I believe the answer is:
chef
'head but no heart' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'he's in charge of restaurant' is the wordplay.
'in' means one lot of letters goes inside another.
'charge of restaurant' becomes 'cf' (I can't justify this - if you can you should believe this answer much more).
'he' put inside 'cf' is 'CHEF'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for chef that I've seen before include "Key kitchen worker" , "Professional cook" , "who has a set of knives?" , "responsible for courses" , "Cooker?" .)