Old and deep tin used as suitable (9)
I believe the answer is:
expedient
'suitable' is the definition.
(I've seen this before)
'old and deep tin used' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'and' says to put letters next to each other.
'used' indicates an anagram.
'deep'+'tin'='deeptin'
'deeptin' with letters rearranged gives 'pedient'.
'ex'+'pedient'='EXPEDIENT'
'as' is the link.
(Other definitions for expedient that I've seen before include "Practical - advantageous" , "Handy" , "Fitting" , "Advantageous in a given situation" , "Opportune" .)