Son finding shopping centre unimportant (5)
I believe the answer is:
small
'unimportant' is the definition.
(similar in meaning)
'son finding shopping centre' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'finding' says to put letters next to each other.
'shopping centre' becomes 'mall' (a mall is a large shopping complex).
's'+'mall'='SMALL'
(Other definitions for small that I've seen before include "Trifle -- half a pint?" , "Young; modest" , "Tiny" , "Petite" , "of little significance" .)