Cow that chap provided to ruler (6)
I believe the answer is:
heifer
'cow' is the definition.
(heifer is a kind of cow)
'chap provided to ruler' is the wordplay.
'chap' becomes 'he'.
'provided' becomes 'if'.
'to ruler' becomes 'er' (abbreviation for Elizabeth Regina).
'he'+'if'+'er'='HEIFER'
'that' is the link.
(Other definitions for heifer that I've seen before include "a little lower" , "Female bovine that has not given birth" , "Farm animal" , "Young cow which has not yet given birth" , "Cow that has borne one or no calves" .)