Nothing right in that lot's speculation (6)
I believe the answer is:
theory
'speculation' is the definition.
(thesaurus)
'nothing right in that lot's' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'right' becomes 'r' (common abbreviation).
'in' is an insertion indicator.
'that lot's' becomes 'they' (I can't justify this - if you can you should believe this answer much more).
'o'+'r'='or'
'or' placed into 'they' is 'THEORY'.
Can you help me to learn more?
(Other definitions for theory that I've seen before include "Proposition, thesis" , "General principles, as opposed to practice" , "Conjectural view" , "Explanatory idea" , "General principle which explains some aspect of the world" .)