Marry posh thicko to get moneyed essentially (5)
I believe the answer is:
unite
'marry' is the definition.
(I've seen this before)
'posh thicko to get moneyed essentially' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'thicko' becomes 'nit'.
'to get' is a charade indicator (letters next to each other).
'essentially' indicates the central letters.
The middle letter of 'moneyed' is 'e'.
'u'+'nit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Join forces" , "ally" , "Come, or join together" , "Join together, become allies" , "Get together" .)