Builder's requisite - a bit of mustard or salt (6)
I believe the answer is:
mortar
'builder's requisite' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a bit of mustard or salt' is the wordplay.
'a bit of' says to take the initial letters.
'salt' becomes 'tar' (both can informally mean a sailor).
The initial letter of 'mustard' is 'm'.
'm'+'or'+'tar'='MORTAR'
Can you help me to learn more?
(Other definitions for mortar that I've seen before include "Brick bonder; weapon" , "Bomb-launching gun" , "Bond for bricks; weapon" , "Gun; brick bonder" , "Bonding material for bricks" .)