Put on protective coat — start in the middle (3)
I believe the answer is:
tar
'put on protective coat' is the definition.
(as in tarmac for a road)
'start in the middle' is the wordplay.
'in the middle' says to take the centre.
The centre of 'start' is 'TAR'.
(Other definitions for tar that I've seen before include "A sailor; a preservative" , "One sails on sea" , "Road surfacing or old sailor" , "Jack ..., British sailor" , "not put on ship in proverbial case of false economy?" .)