Pennine unit intermittently displays tedium (5)
I believe the answer is:
ennui
'tedium' is the definition.
(synonyms)
'pennine unit intermittently' is the wordplay.
'intermittently' indicates alternate letters.
'pennine'+'unit'='pennineunit'
The alternating letters of 'pennineunit' are 'ENNUI'.
'displays' is the link.
(Other definitions for ennui that I've seen before include "having nothing to do" , "Feeling of listlessness" , "Bored listlessness" , "Low interest rate" , "tiredness" .)