Somehow get Len to become amiable (6)
I believe the answer is:
gentle
'amiable' is the definition.
(I've seen this in another clue)
'somehow get len' is the wordplay.
'somehow' indicates anagramming the letters.
'get'+'len'='getlen'
'getlen' anagrammed gives 'GENTLE'.
'to become' acts as a link.
(Other definitions for gentle that I've seen before include "that may help angler catch fish" , "'Do not go ... into that good night'/Thomas" , "Jane Eyre's kind person with a book? -- eternal greed (anag)" , "Mild, not rough" , "Mild, kind, tender" .)