Brood on — is trailing behind in time (7)
I believe the answer is:
agonise
'brood' is the definition.
('brood'->'agonize' is in my internal thesaurus and 'ize'->'ise')
'is trailing behind in time' is the wordplay.
'trailing' says to put letters next to each other.
'behind' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'time' becomes 'age' (an age is a period of time).
'is' put after 'on' is 'onis'.
'onis' going inside 'age' is 'AGONISE'.
'on' acts as a link.
Can you help me to learn more?
(Other definitions for agonise that I've seen before include "Worry greatly (over something); I go sane (anag.)" , "Struggle mentally" , "Worry intensely" , "Fret, worry inordinately" , "Suffer great pain over something" .)