Thoroughly search company doctor (4)
I believe the answer is:
comb
'thoroughly search' is the definition.
'company doctor' is the wordplay.
'company' becomes 'co' (abbreviation).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'co'+'mb'='COMB'
(Other definitions for comb that I've seen before include "Device for tidying one's appearance" , "Fastening spikes" , "Go over" , "Search well; bird's crest" , "Groom the tresses" .)