Maybe thiamine deficiency is dismal (5)
I believe the answer is:
black
'dismal' is the definition.
(thesaurus)
'maybe thiamine deficiency' is the wordplay.
I cannot quite understand how this works, but
'deficiency' could be 'lack' (synonyms) and 'lack' is found in the answer.
A single letter 'b' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for black that I've seen before include "Boycott" , "Side in chess" , "-- Monday" , "second player on board" , "Desperate" .)