Poet who exercised power with a cool head (6)
I believe the answer is:
cowper
'poet' is the definition.
(I've seen this before)
'exercised power with a cool head' is the wordplay.
'exercised' indicates an anagram.
'with' says to put letters next to each other.
'head' suggests taking the first letters.
The initial letter of 'cool' is 'c'.
'power' with letters rearranged gives 'owper'.
'owper' put after 'c' is 'COWPER'.
'who' is the link.
(Other definitions for cowper that I've seen before include "C18 poet; sounds like barrel-maker" , "WRITER" , "linesman" , "William --, English poet" , "Georgian hymn-writer" .)