Extend an arm (5)
I believe the answer is:
reach
'extend an arm' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reach that I've seen before include "range" , "Put out the hand for a part of the river" , "Arrive at (a place)" , "Attain; extend to" , "Come up to - stretch between bends" .)