When working culture’s heartless... (4)
I believe the answer is:
once
'when' is the definition.
(synonyms)
'working culture's heartless' is the wordplay.
'working' becomes 'on'.
'heartless' says to hollow out the word (remove centre letters).
'culture' with its middle removed is 'ce'.
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "On one occasion" , "Some time back" , "Cone (anag.)" , "In times past" , "Singly" .)