One at sea and tense after auction finished early (4)
I believe the answer is:
salt
'auction finished early' is the definition.
I can't tell whether this defines the answer.
'one at sea and tense' is the wordplay.
'one at sea' becomes 'sal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'tense' becomes 't' (abbreviation used in many dictionaries).
'sal'+'t'='SALT'
'after' acts as a link.
Can you help me to learn more?
(Other definitions for salt that I've seen before include "Condiment - old sailor" , "Brackish" , "Sea dog" , "White crystalline substance" , "Mariner; sodium chloride" .)