US agent skirting shaft dropped litter (8)

I believe the answer is:

farrowed

I'm a little stuck... Click here to teach me more about this clue!

'dropped litter' is the definition.
The answer and definition can be both related to bodily activities as well as being past participle verbs.
Perhaps there's an association between them I don't understand?

'us agent skirting shaft' is the wordplay.
'us agent' becomes 'fed' (I've seen this before).
'skirting' is an insertion indicator.
'shaft' becomes 'arrow' (I've seen this before).
'fed' placed around 'arrow' is 'FARROWED'.

Can you help me to learn more?

I've seen this clue in The Guardian.

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