It makes us fit to be of service (6)
I believe the answer is:
usable
'of service' is the definition.
(I've seen this in another clue)
'it makes us fit' is the wordplay.
I cannot quite understand how this works, but
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is present in the answer.
This explanation may well be incorrect...
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for usable that I've seen before include "Of potential utility" , "(Of, eg, tool) employable" , "Like something functional" , "Able to be utilised" , "Fit for service" .)