Principally sensational, except when dull (7)
I believe the answer is:
sunless
'dull' is the definition.
(thesaurus)
'principally sensational except' is the wordplay.
'principally' indicates taking the first letters.
'except' becomes 'unless' (I've seen this before).
The initial letter of 'sensational' is 's'.
's'+'unless'='SUNLESS'
'when' is the link.
(Other definitions for sunless that I've seen before include "Unlit by solar light" , "Dull" , "dim, certainly not bright" , "Offering nothing for a tanner" , "Cloudy" .)