Slender urchin’s first encountered in Euston, straying (7)
I believe the answer is:
tenuous
'slender' is the definition.
(synonyms)
'urchin's first encountered in euston straying' is the wordplay.
'urchin's first' becomes 'u' (1st letter of 'urchin').
'encountered in' indicates putting letters inside.
'straying' indicates anagramming the letters.
'euston' anagrammed gives 'tenuos'.
'u' inserted into 'tenuos' is 'TENUOUS'.
(Other definitions for tenuous that I've seen before include "Lacking substance or significance" , "Flimsy, thin" , "Slender - insubstantial" , "Thin - slight" , "Very weak or slight" .)