Period in which the main issue appears (6)
I believe the answer is:
season
'period' is the definition.
(I know that season is a type of period)
'the main issue appears' is the wordplay.
'the main' becomes 'sea' (the main is an old name for the sea).
'issue' becomes 'son' (issue can mean someone's offspring).
'appears' means one lot of letters go next to another (some letters appear after others).
'sea'+'son'='SEASON'
'in which' is the link.
(Other definitions for season that I've seen before include "Add condiments to for time of year" , "Quarter of the year" , "eg.Summer" , "Add flavouring" , "make things palatable" .)