Deep in thought, reviewing main virtue (10)

I believe the answer is:

ruminative

I'm a little stuck... Click here to teach me more about this clue!

'deep in thought' is the definition.
'ruminative' can be an answer for 'thought' (thesaurus). I am not certain of the 'deep in' bit.

'reviewing main virtue' is the wordplay.
'reviewing' is an anagram indicator (To re view).
'main'+'virtue'='mainvirtue'
'mainvirtue' is an anagram of 'RUMINATIVE'.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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