Unable to sit still, Methuselah has to follow others (7)
I believe the answer is:
restive
'unable to sit still' is the definition.
(restive means fidgety)
'methuselah has to follow others' is the wordplay.
'methuselah has' becomes 'I've' (where the crossword setter is 'Methuselah').
'to follow' means one lot of letters go next to another.
'others' becomes 'rest' (eg the rest of a group).
'ive' after 'rest' is 'RESTIVE'.
(Other definitions for restive that I've seen before include "Impatient - jumpy" , "Resisting control, nervous or impatient" , "Fidgety, uneasy" , "Uneasy, nervous" , "Unsettled" .)