Be under professional examination (5)
I believe the answer is:
probe
'examination' is the definition.
(thesaurus)
'be under professional' is the wordplay.
'under' means one lot of letters go next to another.
'professional' becomes 'pro' ('pro' can be a synonym of 'professional').
'be' after 'pro' is 'PROBE'.
(Other definitions for probe that I've seen before include "Closely enquire into" , "Intrusive device" , "sound" , "Question or examine thoroughly" , "Investigate closely" .)