There's a little sad ending when the gardener kills it (4)
I believe the answer is:
weed
'the gardener kills it' is the definition.
I can't judge whether this definition defines the answer.
'there's a little sad ending' is the wordplay.
'there's a little' becomes 'wee' (I've seen this before).
'ending' suggests the final letters (the letter at the end of the word).
The last letter of 'sad' is 'd'.
'wee'+'d'='WEED'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for weed that I've seen before include "Got rid of water" , "Useless plant - useless person" , "Uncultivated plant" , "Made water" , "Intrusive plant; weak person" .)