Professional examination of the decimal system (7)
I believe the answer is:
medical
'professional examination' is the definition.
'medical' can be an answer for 'examination' (medical examination is a kind of examination). I am not sure about the 'professional' bit.
'decimal system' is the wordplay.
'system' indicates an anagram (another version or system with the same letters).
'decimal' anagrammed gives 'MEDICAL'.
'of the' is the link.
Can you help me to learn more?
(Other definitions for medical that I've seen before include "examination carried out by a doctor" , "Check-up - declaim (anag)" , "Examination of physical health" , "Relating to doctoring" , "Of the science dealing with treatment of disease" .)