Surprise, seeing Sun let loose (7)
I believe the answer is:
startle
'surprise' is the definition.
(thesaurus)
'sun let loose' is the wordplay.
'sun' becomes 'star' (I've seen this before).
'loose' indicates an anagram (letters are loosened from their original positions).
'let' is an anagram of 'tle'.
'star'+'tle'='STARTLE'
'seeing' acts as a link.
(Other definitions for startle that I've seen before include "Frighten suddenly" , "Surprise greatly, alarm" , "Alarm suddenly" , "Cause to feel sudden alarm" , "Make jump" .)