Doctor with a manuscript showing apothecaries' measures (5)
I believe the answer is:
drams
'apothecaries' measures' is the definition.
(I've seen this in another clue)
'doctor with a manuscript' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'with' says to put letters next to each other.
'manuscript' becomes 'ms' (abbreviation for manuscript).
'dr'+'a'+'ms'='DRAMS'
'showing' acts as a link.
(Other definitions for drams that I've seen before include "Wee drinks; weights" , "Wee amounts of whisky" .)