Pair in trouble this month (5)
I believe the answer is:
april
'this month' is the definition.
(April is an example)
'pair in trouble' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'in' indicates putting letters inside.
'trouble' becomes 'ail' (synonyms).
'pr' placed inside 'ail' is 'APRIL'.
(Other definitions for april that I've seen before include "Eliot's 'cruellest month'" , "Woman" , "Fourth month" , "Thirty days in spring" , "Start of the second quarter" .)