Intensify and be louder in a frenzy (8)
I believe the answer is:
redouble
'intensify' is the definition.
(redoubling is a kind of intensifying)
'be louder in a frenzy' is the wordplay.
'in a frenzy' is an anagram indicator.
'be'+'louder'='belouder'
'belouder' with letters rearranged gives 'REDOUBLE'.
'and' is the link.
(Other definitions for redouble that I've seen before include "Produce more" , "Bid" , "Bridge call" , "Be louder to multiply by four - say degree of effort" , "increase fourfold" .)