X introducing sanction of no real value (5)
I believe the answer is:
token
'of no real value' is the definition.
(as in a token gesture)
'x introducing sanction' is the wordplay.
'x' becomes 'ten' (X is the Roman numeral for ten).
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'sanction' becomes 'ok' (to OK something is to approve or sanction it).
'ten' enclosing 'ok' is 'TOKEN'.
(Other definitions for token that I've seen before include "without meaning it" , "Badge; voucher" , "Plastic coin substitute" , "Disk used in place of money" , "Keepsake or memorial as of friendship" .)