Earth is boring planet for adult females (5)

I believe the answer is:

mares

'adult females' is the definition.
(a mare is a female horse)

'earth is boring planet' is the wordplay.
'earth' becomes 'e'.
'is boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'planet' becomes 'mars' (Mars is an example).
'e' placed inside 'mars' is 'MARES'.

'for' is the link.

(Another definition for mares that I've seen is " Female horses".)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app