Buy insurance to protect oneself (4,5)
I believe the answer is:
take cover
'protect oneself' is the definition.
(I've seen this before)
'buy insurance' is the wordplay.
'buy' becomes 'take' (taking is a kind of buying).
'insurance' becomes 'cover' (I've seen this before).
'take'+'cover'='TAKE COVER'
'to' acts as a link.
(Other definitions for take cover that I've seen before include "Seek shelter - buy insurance" , "Find shelter as, say, from bomb" , "Protect oneself" , "Hide" , "Find a place to shelter" .)