In the middle of brief period beset by anxiety (7)
I believe the answer is:
amongst
'in the middle of' is the definition.
(I've seen this before)
'brief period beset by anxiety' is the wordplay.
'brief period' becomes 'mo' (I can't justify this - if you can you should believe this answer much more).
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'anxiety' becomes 'angst' (Angst is an example).
'mo' inserted inside 'angst' is 'AMONGST'.
Can you help me to learn more?
(Other definitions for amongst that I've seen before include "Together with" , "In the midst of" , "Famous novel by John McGahern: '... Women'" , "Having been absorbed, perhaps" , "In the middle of two or more" .)