Mum certainly tense - most likely to withdraw (6)
I believe the answer is:
shyest
'most likely to withdraw' is the definition.
I can't tell whether this definition 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' (grammatical abbreviation).
'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" .)