Two prepare to take the strain (5)
I believe the answer is:
brace
'two' is the definition.
(brace is a kind of two)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for brace that I've seen before include "Support; two" , "Two of a kind" , "Teeth straightener" , "hand tool" , "Tighten, strengthen" .)