Like eggs, half-a-dozen coated in old mould (7)
I believe the answer is:
oviform
'like eggs' is the definition.
(oviform means egg-shaped)
'half-a-dozen coated in old mould' is the wordplay.
'half-a-dozen' becomes 'vi' (Roman numerals for six).
'coated in' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mould' becomes 'form' (synonyms).
'o'+'form'='oform'
'vi' inserted within 'oform' is 'OVIFORM'.
(Other definitions for oviform that I've seen before include "Egg-shaped" , "like Humpty Dumpty" .)