Narrows as a spear may do at the farthest end (6)
I believe the answer is:
tapers
'narrows' is the definition.
(I've seen this before)
'as a spear may do at the farthest end' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
'as' is within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tapers that I've seen before include "Repast with slender candles" , "Becomes gradually narrower as candle may" , "Diminishes gradually" , "Prates about slender candles" , "Narrows towards the end, like a candle" .)