Rambling pair left for thirty days (5)
I believe the answer is:
april
'thirty days' is the definition.
(I've seen this before)
'rambling pair left' is the wordplay.
'rambling' 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 "Spring month" , "Girl" , "Fourth month" , "Thirty days in spring" , "'All in an ... evening' (Katharine Tynan)" .)