Weak sun out, moving over rear of house (7)
I believe the answer is:
tenuous
'weak' is the definition.
('tenuous' can be similar in meaning to 'weak')
'sun out moving over rear of house' is the wordplay.
'moving' indicates an anagram.
'over' indicates putting letters inside.
'rear of' says to take the final letters.
The final letter of 'house' is 'e'.
'sun'+'out'='sunout'
'sunout' is an anagram of 'tnuous'.
'tnuous' enclosing 'e' is 'TENUOUS'.
(Other definitions for tenuous that I've seen before include "Flimsy, thin" , "Sketchy" , "Very thin in form, having little validity" , "Very slight, with little substance or validity" , "Lacking substance or significance" .)