First of Acrosses gets one troubled for a very long time (4)
I believe the answer is:
aeon
'a very long time' is the definition.
(I've seen this before)
'first of acrosses gets one troubled' is the wordplay.
'first of' indicates taking the first letters.
'gets' means one lot of letters go next to another.
'troubled' indicates anagramming the letters.
The initial letter of 'acrosses' is 'a'.
'one' is an anagram of 'eon'.
'a'+'eon'='AEON'
'for' acts as a link.
(Other definitions for aeon that I've seen before include "With lots of time" , "A very long time, an age" , "Eternity" , "Vast age" , "Immeasurably long period of time" .)