It's the herb season we hear (5)
I believe the answer is:
thyme
'it's the herb' is the definition.
(thyme is a kind of herb)
'season we hear' is the wordplay.
'season' becomes 'time' (both can mean a period).
'we hear' indicates a 'sounds like' (homophone) clue.
'time' sounds like 'THYME'.
(Other definitions for thyme that I've seen before include "Aromatic shrub" , "Mint relative" , "Leaves used as seasoning" , "Herb with fragrant leaves - parsley, sage, rosemary and . . . . ." , "one going to pot?" .)