Long period of time before noon (5)
I believe the answer is:
yearn
'long' is the definition.
(I know that long can be written as yearn)
'period of time before noon' is the wordplay.
'period of time' becomes 'year' (year is a kind of period of time).
'before' means one lot of letters go next to another.
'noon' becomes 'n' (abbreviation).
'year'+'n'='YEARN'
(Other definitions for yearn that I've seen before include "Feel desire" , "Desire strongly" , "Have an intense feeling of longing" , "Long - pine" , "wish" .)