Fellow in beret is sad and lonely (6)
I believe the answer is:
bereft
'sad and lonely' is the definition.
I can't judge whether this defines the answer.
'fellow in beret' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'in' is an insertion indicator.
'f' inserted inside 'beret' is 'BEREFT'.
'is' is the link.
Can you help me to learn more?
(Other definitions for bereft that I've seen before include "Deprived, of reason or loved one" , "Grieving from deprivation" , "Cut off - deprived" , "Grief-stricken" , "Having had something precious taken away" .)