Suspect pair left for thirty days (5)
I believe the answer is:
april
'thirty days' is the definition.
(I've seen this before)
'suspect pair left' is the wordplay.
'suspect' indicates anagramming the letters.
'left' becomes 'l' (common abbreviation).
'pair' anagrammed gives 'apri'.
'apri'+'l'='APRIL'
'for' acts as a link.
(Other definitions for april that I've seen before include "March is followed by this" , "Start of the second quarter" , "''All in an ... evening / ... airs were abroad'' (by Katherine Tynan)" , "''... is the cruellest month'' (TS Eliot)" , "The month with All Fools' Day" .)