Doctor, last month, son loses hair (6)
I believe the answer is:
'loses hair' is the definition.
'doctor last month son' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'last month' becomes 'ult' (short for 'last month' in correspondence).
'son' becomes 's'.
(Other definitions for moults that I've seen before include "Sheds feathers or skin", "Shed hair or skin prior to new growth", "Sheds fur or feathers".)