Bolt seat, securing last pieces of track bike build (9)
I believe the answer is:
skedaddle
'bolt' is the definition.
(I've seen this before)
'seat securing last pieces of track bike build' is the wordplay.
'seat' becomes 'saddle' (saddle is a kind of seat).
'securing' indicates putting letters inside.
'last pieces of' suggests the final letters (I've seen 'last piece of' mean this).
The final letters of 'track bike build' are 'ked'.
'saddle' enclosing 'ked' is 'SKEDADDLE'.
(Other definitions for skedaddle that I've seen before include "Depart quickly (colloq.)" , "Rush away" , "Escape" , "Vamoose" , "Depart hurriedly" .)