A small vessel to wrench primarily, big round and tapered at the top (7)
I believe the answer is:
towboat
'a small vessel' is the definition.
'towboat' can be an answer for 'vessel' (towboat is a kind of vessel). I am not certain of the 'a small' bit.
'to wrench primarily big round and tapered at the top' is the wordplay.
'to wrench' becomes 'tow' (I can't explain this - if you can you should give a lot more credence to this answer).
'primarily' suggests taking the first letters.
'round' becomes 'o' (round can mean a round object).
'at the top' says to take the initial letters.
The first letter of 'big' is 'b'.
The first letters of 'and tapered' is 'at'.
'tow'+'b'+'o'+'at'='TOWBOAT'
Can you help me to learn more?
(Other definitions for towboat that I've seen before include "powerful vessel" , "Tug" .)