... heavy, great and eager (6)
I believe the answer is:
gravid
'heavy' is the definition.
(synonyms)
'great and eager' is the wordplay.
'great' becomes 'gr'.
'and' says to put letters next to each other.
'eager' becomes 'avid' (synonyms).
'gr'+'avid'='GRAVID'
(Other definitions for gravid that I've seen before include "(In medicine) pregnant" , "Expecting" , "In an advanced stage of pregnancy" , "Pregnant or heavy" .)