Steadfast astronaut losing limb (6)
I believe the answer is:
strong
'steadfast' is the definition.
(thesaurus)
'astronaut losing limb' is the wordplay.
'astronaut' becomes 'armstrong' (Neil Armstrong).
'losing' indicates named letters should be taken away.
'limb' becomes 'arm' (arm is a kind of limb).
'armstrong' with 'arm' removed is 'STRONG'.
(Other definitions for strong that I've seen before include "Powerful; full-flavoured" , "Potent" , "muscular or concentrated" , "Resilient" , "Vigorous" .)