Thought of a boy-child in a rush (8)
I believe the answer is:
reasoned
'thought' is the definition.
(reasoning is a kind of thinking)
'a boy-child in a rush' is the wordplay.
'boy child' becomes 'son' (I've seen this in another clue).
'in' is an insertion indicator.
'a rush' becomes 'reed' (plants found by water).
'a'+'son'='ason'
'ason' inserted into 'reed' is 'REASONED'.
'of' acts as a link.
(Other definitions for reasoned that I've seen before include "Worked out through logic and sense" , "Performed brain operations" , "Dear ones (anag)" , "Argued or deduced" , "clear" .)