Hen party's ending in reverse of true (5)
I believe the answer is:
layer
'hen' is the definition.
(something which lays - eggs in this case)
'party's ending in reverse of true' is the wordplay.
'ending' suggests the final letters (the ending of the word).
'in' means one lot of letters goes inside another.
'reverse of' says the letters should be written backwards.
'true' becomes 'real' (synonyms).
The final letter of 'party' is 'y'.
'real' reversed gives 'laer'.
'y' inserted within 'laer' is 'LAYER'.
(Other definitions for layer that I've seen before include "Thin sheet - productive hen" , "One producing eggs" , "Stratum; chicken" , "Tier or thickness - or a hen?" , "Single thickness, tier" .)