Historic state capital certain to retain an opening for them (5,2)
I believe the answer is:
santa fe
'historic state capital' is the definition.
'Santa Fe' can be an answer for 'capital' (state capital of New Mexico). I'm not sure about the 'historic state' bit.
'certain to retain an opening for them' is the wordplay.
'certain' becomes 'safe' (I am not sure about this - if you are sure you should believe this answer much more).
'to retain' indicates putting letters inside.
'opening for' says to take the initial letters.
The first letter of 'them' is 't'.
'an'+'t'='ant'
'safe' placed around 'ant' is 'SANTA FE'.
Can you help me to learn more?
(Other definitions for santa fe that I've seen before include "Capital city of New Mexico" , "Capital of New Mexico state" , "US state capital" , "US city" .)