Before lunch, finally manage to go back in time (5)
I believe the answer is:
epoch
'time' is the definition.
(I know that Epoch is a type of time)
'before lunch finally manage to go back' is the wordplay.
'before' says to put letters next to each other.
'finally' indicates one should take the final letters.
'manage' becomes 'cope' (manage can mean to cope or do well).
'to go back' shows that the letters should be reversed in order.
The final letter of 'lunch' is 'h'.
'cope' reversed gives 'epoc'.
'h' after 'epoc' is 'EPOCH'.
'in' acts as a link.
(Other definitions for epoch that I've seen before include "Age, time span" , "A particular period of history" , "Historical age" , "Significant period" , "Aeon" .)