Three times provided by the head of Harrow in an instant (6)
I believe the answer is:
thrice
'three times' is the definition.
('thrice' can be a synonym of 'three times')
'the head of harrow in an instant' is the wordplay.
'the head of' says to take the initial letters.
'in' indicates putting letters inside.
'an instant' becomes 'trice' ('trice' can be a synonym of 'instant').
The initial letter of 'harrow' is 'h'.
'h' inserted inside 'trice' is 'THRICE'.
'provided by' acts as a link.
(Other definitions for thrice that I've seen before include "Archaic word for three times" , "On three occasions" , "as it can be found in little titbits" , "Old usage for three times" , "In threefold degree" .)