Think “Heavens! Denis has broken in!” (8)
I believe the answer is:
consider
'think' is the definition.
(considering is a kind of thinking)
'heavens denis has broken in' is the wordplay.
I cannot really see how this works, but
an anagram of 'denis' is 'nside' which is present in the answer.
The remaining letters 'cor' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for consider that I've seen before include "Reckon, deem" , "Go over something in one's mind" , "Ponder on" , "view" , "Suppose" .)