Henry Ford's first drink, relatively modest (4)
I believe the answer is:
half
'drink relatively modest' is the definition.
(a half pint)
'henry ford's first' is the wordplay.
'henry' becomes 'hal'.
'ford's first' becomes 'f' (1st letter of 'ford').
'hal'+'f'='HALF'
(Other definitions for half that I've seen before include "Wife the better one (joc.)" , "little drink" , "Two of these to the pint" , "Portion of bread (better than none?)" , "alcoholic drink" .)