Thus be dependent, distressingly! (6)
I believe the answer is:
sorely
'distressingly' is the definition.
(thesaurus)
'thus be dependent' is the wordplay.
'thus' becomes 'so' (synonyms).
'be dependent' becomes 'rely' (I've seen this before).
'so'+'rely'='SORELY'
(Other definitions for sorely that I've seen before include "Acutely; very much" , "Extremely, severely" , "Painfully" .)