Call investigator a novice (4)
I believe the answer is:
dial
'call' is the definition.
(I've seen this in other clues)
'investigator a novice' is the wordplay.
'investigator' becomes 'di' (abbreviation for detective inspector).
'novice' becomes 'l' (abbreviation for Learner e.g. on UK cars).
'di'+'a'+'l'='DIAL'
(Other definitions for dial that I've seen before include "Select a telephone number" , "Ring the clock-face" , "Coil of yarn" , "Part of old telephone" , "Face of a timepiece" .)