Regiment kept in the dark loses heart (4)
I believe the answer is:
unit
'regiment' is the definition.
(thesaurus)
'in the dark loses heart' is the wordplay.
'in the dark' becomes 'unlit' (I've seen this before).
'loses heart' says to hollow out the word (remove centre letters).
'unlit' with its middle removed is 'UNIT'.
'kept' acts as a link.
(Other definitions for unit that I've seen before include "Single person - group of people" , "Standard for measuring" , "One - kitchen cabinet" , "A single, complete item" , "Individual component" .)