Question to policeman about heart of investigation (5)
I believe the answer is:
topic
'question' is the definition.
(I've seen this before)
'to policeman about heart of investigation' is the wordplay.
'policeman' becomes 'pc'.
'about' indicates putting letters inside.
'heart of' indicates the central letters.
The centre of 'investigation' is 'i'.
'pc' placed around 'i' is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject (eg, of text)" , "Subject (of, eg, speech)" , "Subject matter, theme" , "Matter for an argument" , "'Theme, subject (5)'" .)