Pharmacist beginning to tire after endless shift (7)
I believe the answer is:
chemist
'pharmacist' is the definition.
(synonyms)
I can't explain the remainder of the clue.
(Other definitions for chemist that I've seen before include "Shop for drugs" , "Pharmacist, apothecary" , "Drug seller" , "Scientist; type of shop" , "Seller of medicines and toiletries" .)