Dreamer is humorous, having a large expression (6,5)
I believe the answer is:
walter mitty
'dreamer' is the definition.
(I've seen this before)
'humorous having a large expression' is the wordplay.
'humorous' becomes 'witty' (similar in meaning).
'having' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'expression' becomes 'term' (I've seen this in another clue).
'a'+'l'+'term'='alterm'
'witty' enclosing 'alterm' is 'WALTER MITTY'.
'is' is the link.
(Other definitions for walter mitty that I've seen before include "fanciful sort" , "Dreamer" , "Thurber character - one who fantasises" , "Thurber's fictional fantasist" , "fictional character" .)