Unfortunately, true love's not very steadfast (8)
I believe the answer is:
resolute
'steadfast' is the definition.
(similar in meaning)
'unfortunately true love's not very' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'not' suggests deleting specific letters.
'very' becomes 'v' (common abbreviation).
'true'+'loves'='trueloves'
'trueloves' with 'v' taken away is 'trueloes'.
'trueloes' is an anagram of 'RESOLUTE'.
(Other definitions for resolute that I've seen before include "Showing great determination, fully committed" , "Admirably purposeful" , "'Determined, dead set on (8)'" , "Determined, intent" , "in a determined manner" .)