Disloyal squad one left with regret (6)
I believe the answer is:
untrue
'disloyal' is the definition.
(similar in meaning)
'squad one left with regret' is the wordplay.
'squad' becomes 'unit' (I've seen this before).
'one' becomes 'I' (Roman numeral).
'left' indicates named letters should be taken away.
'with' says to put letters next to each other.
'regret' becomes 'rue' (synonyms).
'unit' with 'i' taken out is 'unt'.
'unt'+'rue'='UNTRUE'
(Other definitions for untrue that I've seen before include "Incorrect or false" , "Not accurate or veracious" , "False or crooked" , "Not factual or honest" , "Not level" .)