Worried when extremists tweeted about Russian currency (8)
I believe the answer is:
troubled
'worried' is the definition.
(synonyms)
'extremists tweeted about russian currency' is the wordplay.
'extremists' says to hollow out the word (remove centre letters).
'about' is an insertion indicator.
'russian currency' becomes 'rouble' (I've seen this before).
'tweeted' with its middle removed is 'td'.
'td' enclosing 'rouble' is 'TROUBLED'.
'when' is the link.
(Other definitions for troubled that I've seen before include "Disturbed, bothered" , "Bothered, perturbed" , "Caused distress to" , "Anxious" , "Showing anxiety" .)