Deposit, say, posh pharmacist in Hebridean stack (4,3)
I believe the answer is:
lump sum
'deposit say' is the definition.
(a deposit might be paid as a lump sum)
'posh pharmacist in hebridean stack' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'pharmacist' becomes 'MPS' (Member of the Pharmaceutical Society).
'in' is an insertion indicator.
'hebridean stack' becomes 'lum' (Scottish term for a chimney).
'u'+'mps'='umps'
'umps' put inside 'lum' is 'LUMP SUM'.
(Other definitions for lump sum that I've seen before include "cash payout" , "Single payment as opposed to a number of smaller ones" , "as pension may be part-paid?" , "Substantial payment" , "Large single payment" .)