They regularly exercise right to be over-excited (5)
I believe the answer is:
hyper
'be over-excited' is the definition.
(I've seen this before)
'they regularly exercise right' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'exercise' becomes 'pe' (Physical Education).
'right' becomes 'r' (common abbreviation).
The alternate letters of 'they' are 'hy'.
'hy'+'pe'+'r'='HYPER'
'to' acts as a link.
(Other definitions for hyper that I've seen before include "extraordinarily energetic" , "Unusually energetic" , "Prefix meaning 'Excessive'" , "Prefix meaning excessively, perhaps active" , "Unable to sit still" .)