Poser: one surely won't be missed (6)
I believe the answer is:
sitter
'poser' is the definition.
(I know that poser is a type of poser)
'be missed' is the wordplay.
'be' becomes 'sit' (sitting is a kind of being).
'missed' becomes 'ter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sit'+'ter'='SITTER'
'one surely won't' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sitter that I've seen before include "One minds the baby" , "Child-minder of sedentary disposition" , "Easy shot or catch" , "A poser" , "easy target" .)