Growth of delivery service force (7)
I believe the answer is:
upsurge
'growth' is the definition.
(I've seen this before)
'delivery service force' is the wordplay.
'delivery service' becomes 'ups' (United Parcel Service, deliveries company).
'force' becomes 'urge' (I've seen this in another clue).
'ups'+'urge'='UPSURGE'
'of' acts as a link.
(Other definitions for upsurge that I've seen before include "Effect of groundswell" , "Sudden or abrupt strong increase" , "Sudden large increase" , "growth" , "A sudden rise, as in business" .)