Most retiring, indeed, when in bed (6)
I believe the answer is:
coyest
'most retiring' is the definition.
(coy can mean retiring or shy)
'indeed when in bed' is the wordplay.
'indeed' becomes 'yes' (agreeing).
'when in' is an insertion indicator.
'bed' becomes 'cot' (cot is a kind of bed).
'yes' put within 'cot' is 'COYEST'.
(Other definitions for coyest that I've seen before include "Most modest" , "Most reluctant" .)