Hard time of day is the most intense part (6)
I believe the answer is:
height
'the most intense part' is the definition.
(e.g. the height of the tourist season)
'hard time of day' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'time of day' becomes 'eight' (8am).
'h'+'eight'='HEIGHT'
'is' is the link.
(Other definitions for height that I've seen before include "Hill, elevation" , "Distance form top to bottom" , "Measure it from top to bottom" , "Loftiness" , "Dimension upwards" .)