Gains the lead from, but it doesn't last (6)
I believe the answer is:
passes
'gains' is the definition.
The answer and definition can be both related to bodily activities as well as being verbs in their -s form.
Perhaps they are linked in a way I don't understand?
'lead from but it doesn't last' is the wordplay.
I cannot quite see how this works, but
'lead' could be 'pass' (synonyms) and 'pass' is present in the answer.
'from' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'the' acts as a link.
Can you help me to learn more?
(Other definitions for passes that I've seen before include "Advances" , "Achieves the required standard in a test" , "Routes through mountains" , "Documents indicating permission" , "Dies, euphemistically" .)