Be concerned about daughter in service unit (5)
I believe the answer is:
cadre
'service unit' is the definition.
(a military unit)
'be concerned about daughter' is the wordplay.
'be concerned' becomes 'care' (I've seen this before).
'about' means one lot of letters goes inside another.
'daughter' becomes 'd'.
'care' enclosing 'd' is 'CADRE'.
'in' is the link.
(Other definitions for cadre that I've seen before include "Small (military) unit capable of expansion when necessary" , "team" , "Raced about in military group" , "Nucleus of servicemen" , "Nucleus of key personnel" .)