Most important horse is fed carefully at first (5)
I believe the answer is:
focal
'most important' is the definition.
'horse is fed carefully at first' is the wordplay.
'horse' becomes 'foal' (I've seen this before).
'is fed' indicates putting letters inside (one word is \'fed\' to another).
'at first' says to take the initial letters.
The first letter of 'carefully' is 'c'.
'foal' enclosing 'c' is 'FOCAL'.
(Other definitions for focal that I've seen before include "- length characterises a lens" , "Things are concentrated at such a point" , "Relating to a centre of interest" , "Type of length or point" , "Relating to point of concentration" .)