Now and then, Seuss writes with energy and tension (8)
I believe the answer is:
suspense
'tension' is the definition.
(I've seen this before)
'now and then seuss writes with energy' is the wordplay.
'now and then' indicates alternate letters (letters taken on a regular basis, now and then).
'writes' becomes 'pens' ('pen' can be a synonym of 'write').
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
The alternate letters of 'seuss' are 'sus'.
'sus'+'pens'+'e'='SUSPENSE'
'and' is the link.
(Other definitions for suspense that I've seen before include "Tense expectation" , "Feeling of excitement or anxiety about what might happen" , "Uneasy uncertainty" , "Alfred Hitchcock was the master of it" , "Excitement and uncertainty about an outcome" .)