Ruse mentioned for a period (5)
I believe the answer is:
while
'a period' is the definition.
(while is a kind of period)
'ruse mentioned' is the wordplay.
'ruse' becomes 'wile' (I've seen this before).
'mentioned' indicates a 'sounds like' (homophone) clue.
'wile' sounds like 'WHILE'.
'for' acts as a link.
(Other definitions for while that I've seen before include "During the time that, as long as" , "Space of time" , "Whereas" .)