Difficult to carry on this evening (7)
I believe the answer is:
tonight
'this evening' is the definition.
(synonyms)
'difficult to carry on' is the wordplay.
'difficult' becomes 'tight' (similar in meaning).
'to carry' indicates putting letters inside.
'tight' going around 'on' is 'TONIGHT'.
(Other definitions for tonight that I've seen before include "Selection from West Side Story" , "the coming time for retirement" , "Hotting up for after sundown" , "after supper?" , "On tight (anag.)" .)