Finally the son has lark — one getting away (7)
I believe the answer is:
escaper
'one getting away' is the definition.
(someone making an escape)
'finally the son has lark' is the wordplay.
'finally' says to take the final letters.
'son' becomes 's' (genealogical abbreviation for son).
'has' is a charade indicator (letters next to each other).
'lark' becomes 'caper' (I've seen this before).
The final letter of 'the' is 'e'.
'e'+'s'+'caper'='ESCAPER'
(Other definitions for escaper that I've seen before include "One runs away" , "One making a getaway" , "One who gets away" , "it's no longer caged" , "one getting away" .)