Watchword of guy having painting put up (6)
I believe the answer is:
mantra
'watchword' is the definition.
(I've seen this before)
'guy having painting put up' is the wordplay.
'guy' becomes 'man' (I've seen this before).
'having' says to put letters next to each other.
'painting' becomes 'art' (I've seen this before).
'put up' is a reversal indicator.
'art' written backwards gives 'tra'.
'man'+'tra'='MANTRA'
'of' acts as a link.
(Other definitions for mantra that I've seen before include "words of comfort repeatedly" , "Word or phrase repeated as aid to concentration" , "Incantation, Hindu psalm" , "Buddhist aid to concentration" , "Slogan; refrain" .)