Plead with sentenced criminal to take heart in tough part of sentence (9,6)

'sentence' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?

'plead with sentenced criminal to take heart in tough' is the wordplay.
'with' says to put letters next to each other.
'criminal' indicates anagramming the letters.
'to take' means one lot of letters goes inside another (inserted letters are taken in).
'heart in' means to look at the middle letters.
The middle of 'tough' is 'u'.
'plead'+'sentenced'='pleadsentenced'
'pleadsentenced' with letters rearranged gives 'dependentclase'.
'dependentclase' placed around 'u' is 'DEPENDENT CLAUSE'.

'part of' acts as a link.

Can you help me to learn more?