Crawling son, small (4)
I believe the answer is:
slow
'crawling' is the definition.
('slow' can be similar in meaning to 'crawling')
'son small' is the wordplay.
'son' becomes 's'.
'small' becomes 'low' ('low' can be a synonym of 'small').
's'+'low'='SLOW'
(Other definitions for slow that I've seen before include "Intellectually dull" , "Dilatory" , "Crawling" , "Not getting a move on" , "Reluctant" .)