Seize end of big flipping obstacle (4)
I believe the answer is:
grab
'seize' is the definition.
(grabbing is a kind of seizing)
'end of big flipping obstacle' is the wordplay.
'end of' indicates one should take the final letters.
'flipping' is a reversal indicator.
'obstacle' becomes 'bar' (bar can mean an obstacle or blockage).
The final letter of 'big' is 'g'.
'bar' back-to-front is 'rab'.
'g'+'rab'='GRAB'
(Other definitions for grab that I've seen before include "Grasp roughly" , "Take quickly" , "'Seize, snatch (4)'" , "Seize suddenly" , "Seize rudely" .)