Mum certainly tense - most likely to withdraw (6)
I believe the answer is:
shyest
'most likely to withdraw' is the definition.
I can't judge whether this defines the answer.
'mum certainly tense' is the wordplay.
'mum' becomes 'sh' (both can be used to tell someone to be quiet).
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'tense' becomes 't' (abbreviation used in many dictionaries).
'sh'+'yes'+'t'='SHYEST'
Can you help me to learn more?
(Other definitions for shyest that I've seen before include "Most retiring" , "shrinking the most?" , "Most timid" , "Most reticent" , "Most bashful or self-conscious" .)