Herb keeps step in modern times (5,3)
I believe the answer is:
space age
'modern times' is the definition.
(I've seen this before)
'herb keeps step' is the wordplay.
'herb' becomes 'sage' (sage is a kind of herb).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'step' becomes 'pace' (synonyms).
'sage' placed around 'pace' is 'SPACE-AGE'.
'in' acts as a link.
(Other definitions for space age that I've seen before include "Now we can travel so far!" , "progress since nineteen fifty seven" , "(Technologically) highly advanced" , "These days" , "extended travel time" .)