The bachelor beside the infant (4)
I believe the answer is:
baby
'infant' is the definition.
(I know that infant can be written as baby)
'the bachelor beside' is the wordplay.
'the bachelor' becomes 'BA' (abbreviation for Bachelor of Arts degree).
'beside' becomes 'by'.
'ba'+'by'='BABY'
'the' is the link.
(Other definitions for baby that I've seen before include "Infancy" , "invention" , "Timid person; very small" , "about to howl?" , "New-born child" .)