Development of son I see, at least challenging (7)
I believe the answer is:
easiest
'least challenging' is the definition.
(I've seen this in another clue)
'development of son i see at' is the wordplay.
'development of' indicates anagramming the letters (letters developed in another way).
'son' becomes 's' (genealogical abbreviation for son).
's'+'i'+'see'+'at'='siseeat'
'siseeat' is an anagram of 'EASIEST'.
(Other definitions for easiest that I've seen before include "Most convenient" , "Requiring the least amount of effort" , "Least difficult" , "Most effortless" , "most relaxed" .)