Follow to the letter (4)
I believe the answer is:
obey
'follow to the letter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for obey that I've seen before include "Do as one's told" , "Act in accordance with (law)" , "Toe the line" , "Submit to authority" , "Carry out (an order)" .)