Nervy movements going round bog? Toughen up! (7)
I believe the answer is:
stiffen
'toughen up' is the definition.
(to stiffen is to toughen up or become firmer)
'nervy movements going round bog?' is the wordplay.
'nervy movements' becomes 'fits' (as in a fit of nerves).
'going round' shows that the letters should be reversed in order.
'bog?' becomes 'fen' (I've seen this in another clue).
'fits' back-to-front is 'stif'.
'stif'+'fen'='STIFFEN'
(Other definitions for stiffen that I've seen before include "Make rigid or inflexible" , "Steel" , "Add starch" , "Reinforce" , "Make hard" .)