Honour nurse, not able to express thanks (7)
I believe the answer is:
ennoble
'honour' is the definition.
(I know that ennoble is a more specific form of honour)
'nurse not able to express thanks' is the wordplay.
'nurse' becomes 'EN' (abbreviation for enrolled nurse).
'to express' suggests deleting specific letters.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'not'+'able'='notable'
'notable' with 'ta' taken away is 'noble'.
'en'+'noble'='ENNOBLE'
(Other definitions for ennoble that I've seen before include "Raise status" , "Confer dignity on" , "Turn to peer" , "Give peerage to" , "Bestow high rank upon" .)