a great deal is an adverb meaning "to a very great degree or extent". It carries 2 recorded senses.
to a very great degree or extent
“I feel a lot better”
“we enjoyed ourselves very much”
“she was very much interested”
“this would help a great deal”
frequently or in great quantities
“I don't drink much”
“I don't travel much”
Definitions, synonyms and semantic relations for a great deal come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.