Small sailor has a heavenly body! (4)
I believe the answer is:
star
'a heavenly body' is the definition.
(star is a kind of heavenly body)
'small sailor' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sailor' becomes 'tar' (slang term for a sailor).
's'+'tar'='STAR'
'has' is the link.
(Other definitions for star that I've seen before include "Celebrated actor - heavenly body!" , "Major" , "Polaris, possibly" , "Asterisk" , "- - of Bethlehem" .)