Investigation of nick, police cleared guards (5)
I believe the answer is:
probe
'investigation' is the definition.
(probe can mean an investigation or inquiry)
'nick police cleared guards' is the wordplay.
'nick' becomes 'rob' (both can mean to steal).
'cleared' means to remove the middle letters (the inside letters are cleared out).
'guards' indicates putting letters inside (inserted letters are guarded).
'police' with its centre taken out is 'pe'.
'rob' inserted inside 'pe' is 'PROBE'.
'of' is the link.
(Other definitions for probe that I've seen before include "(Crime) investigation" , "Intrusive device" , "Inquiry" , "sound" , "Closely enquire into" .)