Jockey is trained to keep first of mares in control (10)
I believe the answer is:
administer
'control' is the definition.
(I know this)
'jockey is trained to keep first of mares' is the wordplay.
'jockey' indicates an anagram (the letters scramble or jockey around).
'to keep' indicates putting letters inside.
'first of mares' becomes 'm' (1st letter of 'mares').
'is'+'trained'='istrained'
'istrained' anagrammed gives 'adinister'.
'adinister' going around 'm' is 'ADMINISTER'.
'in' is the link.
(Other definitions for administer that I've seen before include "Handle" , "man?" , "Deal out (punishment)" , "Oversee" , "Run" .)