Unfortunately I'm old -- I sustain pretence (13)

I believe the answer is:

dissimulation

'pretence' is the definition.

'unfortunately i'm old i sustain' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'im'+'old'+'i'+'sustain'='imoldisustain'
'imoldisustain' is an anagram of 'DISSIMULATION'.

(Another definition for dissimulation that I've seen is " Hypocrisy".)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app