Photo leading to surprised comment about large instrument (7)
I believe the answer is:
piccolo
'instrument' is the definition.
(piccolo is a kind of instrument)
'photo leading to surprised comment about large' is the wordplay.
'photo' becomes 'pic' (I've seen this before).
'leading to' says to put letters next to each other (some letters go before or lead others).
'surprised comment' becomes 'coo' (expression of surprise).
'about' means one lot of letters goes inside another.
'large' becomes 'l'.
'coo' going around 'l' is 'colo'.
'pic'+'colo'='PICCOLO'
(Other definitions for piccolo that I've seen before include "Smallest of the flute family" , "Shrill woodwind instrument" , "Instrument (meaning "little")" , "Little flute" , "Part of 14 [ORCESTRA]" .)